Feeding and Infant Sleep | Encyclopedia MDPI

Feeding and Infant Sleep: Comparison

Please note this is a comparison between Version 1 by Xiaoxi Fu and Version 2 by Jason Zhu.

Inconsistent conclusions from infant sleep and feeding studies may influence parents feeding-related decisions. Exclusively breastfed infants (≤6 months-of-age) had a greater number of night wakings, but most studies reported no difference in night-time and 24 h sleep duration compared to formula-fed infants. However, after 6 months-of-age, most studies reported breastfed infants to sleep less in the night-time and over 24 h compared to formula-fed infants. Furthermore, studies reported no association between the timing of introduction to complementary foods and infant sleep duration (<12 months-of-age).

infant
sleep
feeding mode
complementary feeding
sleep duration
night wakings
night-time sleep

1. Introduction

Feeding type and sleep patterns are dynamic processes throughout the first year of life and have significant effects on health and development ^[1][2][1,2]. The World Health Organization (WHO) recommends exclusive breastfeeding for the first completed six months of life, with the introduction of complementary foods after six months-of-age [2]. Breastfeeding provides short- and long-term benefits to both infants and mothers, including protection against acute and chronic disorders among infants and as they grow older ^[3][4][3,4]. Sleep during the first year of life is especially important due to the rapid changes that occur in the consolidation of sleep/wake patterns ^[5][6][7][5,6,7]. The quantity and quality of an infant’s sleep are associated with cognitive function such as the development of memory and language [8], and the ability to learn ^[9][10][9,10]. In addition, insufficient sleep and sleep problems have been associated with later obesity [11] and behavioral issues such as tantrums and other behavioral management problems [12]. Frequent and extended night wakings, one of the most common infant-sleep-related problems, has also been shown to affect infant health and development ^[13][14][15][13,14,15]. Therefore, sufficient sleep during infancy is a priority [8] and is often one of the main issues reported by new parents, with frequent parental night wakings shown to affect parent mood and function ^{[13][14][15][16]}[13,14,15,16]. An estimated 20–30% of children experience sleep problems during the first three years of life according to a cross-sectional study conducted in New Zealand and Australia, with one-third of parents reporting their infants as having a sleep problem [17].

A number of studies have examined the relationship between sleep and feeding among infants. The timing of introduction to complementary foods has been associated with infant sleep patterns, with breastfeeding reportedly playing a role in increasing sleep disturbances ^{[17][18][19][20][21]}[17,18,19,20,21], while other studies have not found such significance ^[22][23][22,23]. The potential association between type of milk feeding or the timing of introduction to complementary foods and sleep may drive parental beliefs that early introduction to complementary foods or changes to the type of milk feeding, contrary to current recommendations [2], may improve their infants’ sleep patterns ^[19][24][19,24]. The lack of consistency of the available evidence could be a source of confusion for parents, thereby affecting feeding-related decisions during the first year of life.

2. Current Studies

2.1. Type of Milk Feeding and Infant Sleep

21 studies reported on type of milk feeding in relation to infant sleep patterns as shown in table 1. The type of milk feeding was reported prospectively by parents or caregivers through questionnaires ^{[17][19][23][25][26][27][28]}[17,19,23,30,31,35,38], interviews ^[29][30][32,46], feeding logs ^[31][32][33][36,37,45], and by maternal self-report ^{[34][35][36][37][38][39]}[33,34,39,40,43,44] except for one study ^[40][41], that assessed type of milk feeding retrospectively through a questionnaire. Two studies ^[41][42][29,42] did not specify their assessment methods.

Table 1. Type of milk feeding among infants aged ≤ 6 months, >6 months, and 0–12 months in relation to sleep variables including 24 h sleep duration, total night-time sleep, night-waking frequency, duration of night wakings, longest sleep period, and sleep onset latency.

24 h Sleep Duration
Author, Year	Infant Age at Assessment/Assessment Frequency	≤6 Months		>6 Months	0–12 Months	Statistics	¹	Quality Rating	²
Author, Year	Infant Age at Assessment/Assessment Frequency	EBF vs FF	¹	BF vs. FF	¹	Statistics	¹	Quality Rating	²	BF vs. FF	¹	BF vs. FF	¹
Berger et al., 2017 ^[32]	Berger et al., 2017 [37]	16 weeks		BF vs. FF mean ± SE (h) 12.95 ± 0.51 vs. 11.43 ± 0.53,	p	= 0.047			t	-test	6
Butte et al., 1992 ^[41]	Butte et al., 1992 [29]	17 weeks		BF vs. FF mean ± SD (h) 13.2 ± 2.3 vs.13.3 ± 0.9,	p	> 0.05			t	-test Regression	5
Figueiredo et al., 2017 ^[28]	Figueiredo et al., 2017 [38]	2, 13, 26 weeks	EBF vs. FF mean ± SD (h) 2 weeks 13.43 ± 2.34 vs. 12.29 ± 2.27,	p	> 0.05 13 weeks 13.05 ± 1.87 vs. 12.87 ± 2.44,	p	> 0.05 26 weeks 12.37 ± 1.76 vs. 12.79 ± 1.05,	p	> 0.05	BF vs. FF mean ± SD (h) 2 weeks 12.18 ± 3.02 vs. 12.29 ± 2.27,	p	> 0.05 13 weeks 12.41 ± 2.21 vs. 12.87 ± 2.44,	p	> 0.05 26 weeks 12.73 ± 1.48 vs. 12.79 ± 1.05,	p	> 0.05			Multivariate Analyses of Chi MANCOVA	³	5
Kaley et al., 2012 ^[26]	Kaley et al., 2012 [31]	4–10 weeks		BF vs. FF Total sleep not assoc. with feeding,	p	> 0.05			Correlation ANOVA	4
Lee et al., 2000 ^[29]	Lee et al., 2000 [32]	2–17 weeks	EBF vs. FF mean ± SD (min) 902.4 ± 119.1 vs. 854.8 ± 130.7,	p	< 0.01				Unpaired	t	-test	4
Quillin et al., 2004 ^[35]	Quillin et al., 2004 [34]	4 weeks		BF	vs.	FF	mean ± SD (h) 13.1 ± 1.4 vs. 14.4 ± 1.1,	p	= 0.006			t	-test	4
Tikotzky et al., 2011 ^[39]	Tikotzky et al., 2011 [44]	26 weeks	EBF vs. FF No assoc. between total sleep and EBF (r = 0.15,	p	> 0.05)				Spearman rho correlations	6
Nevarez et al., 2010 [23]	26, 52 weeks	⁴		BF +/− food vs. FF +/− food Bivariate 26 weeks β = 0.05 (95%CI: −0.14 to 0.24),	p	> 0.05 Multivariate 26 weeks β = −0.15 (95%CI: −0.37 to 0.07),	p	> 0.05	BF +/− food vs. FF +/− food Bivariate 52 weeks β = 0.02 (95%CI: −0.17 to 0.20),	p	> 0.05 Multivariate 52 weeks is β = −0.17 (95%CI: −0.37 to 0.03),	p	> 0.05		Bivariate Multivariate linear regression	⁵	5
Heinig et al., 1993 [42]	39 weeks			BF +/− food vs. FF +/− food 24 h sleep at 39 weeks greater in FF compared to BF grps,	p	< 0.05		t	-test	4
Morgan et al., 2004 ^[30]	Morgan et al., 2004 [46]	39 weeks	⁴			BF +/− food vs. FF +/− food mean ± SE (h) 11.2 ± 0.1 vs. 11.4 ± 0.6,	p	= 0.01	⁶		ANCOVA	⁷	5
Huang et al., 2016 ^[36]	Huang et al., 2016 [39]	0–34 weeks				BF +/− food vs. FF +/− food BF 2.1% lower (30 min less) 24 h sleep % than FF,	p	= 0.0009	Multilevel mixed models	4
Total Night-Time Sleep
Author, Year	Infant Age at Assessment/Assessment Frequency	≤6 Months		>6 Months	0–12 Months	Statistics ¹	Quality Rating ²
Author, Year	Infant Age at Assessment/Assessment Frequency	EBF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	Statistics ¹	Quality Rating ²
Berger et al., 2017 ^[32]	Berger et al., 2017 [37]	16 weeks		BF vs. FF mean ± SE (h) 9.50 ± 0.38 vs. 7.33 ± 0.39,	p	< 0.0001			t	-test	6
Butte et al., 1992 ^[41]	Butte et al., 1992 [29]	17 weeks		BF vs. FF mean ± SD (h) 8.2 ± 1.6 vs. 9.9 ± 1.4,	p	< 0.04			t	-test Regression	5
Figueiredo et al., 2017 ^[28]	Figueiredo et al., 2017 [38]	2, 13, 26 weeks	EBF vs. FF mean ± SD (h) 2 weeks 7.08 ± 1.33 vs. 6.34 ± 1.21,	p	> 0.05 13 weeks 8.06 ± 1.30 vs. 8.27 ± 1.35,	p	> 0.05 26 weeks 8.29 ± 1.36 vs. 8.29 ± 1.07,	p	> 0.05	BF vs. FF mean ± SD (h) 2 weeks 6.77 ± 1.55 vs. 6.34 ± 1.21,	p	> 0.05 13 weeks 8.12 ± 1.22 vs. 8.27 ± 1.35,	p	> 0.05 26 weeks 8.93 ± 1.21 vs. 8.29 ± 1.07,	p	> 0.05			Multivariate Analyses of Chi MANCOVA	³	5
Kaley et al., 2012 ^[26]	Kaley et al., 2012 [31]	4–10 weeks		BF vs. FF NTS duration not assoc. with feeding,	p	> 0.05			Correlation ANOVA	4
Quillin et al., 1997 ^[34]	Quillin et al., 1997 [33]	4 weeks		BF vs. FF BF infants slept less at night than FF infants. F(1,39) = 4.925,	p	< 0.05			ANOVA-two-way analysis of variance	3
Quillin et al., 2004 ^[35]	Quillin et al., 2004 [34]	4 weeks		BF vs. FF mean ± SD (h) 6.4 ±1.0 vs. 6.4 ± 0.8,	p	> 0.05			t	-test	4
Rudzik et al., 2018 ^[38]	Rudzik et al., 2018 [43]	4,6,8,10,12,14,16, 18 weeks	EBF vs. FF Actigraph report No difference between grps for NTS at 2, 6, 8, 10, 12, 14, 16, 18 weeks,	p	> 0.05				t	-test	3
Yoshida et al., 2015 ^[33]	Yoshida et al., 2015 [45]	13, 17 weeks	EBF vs. FF STN (6 h criterion): 33% vs. 67%				Multiple linear regression	5
Pennestri et al., 2018 ^[40]	Pennestri et al., 2018 [41]	26, 52 weeks		BF +/− food vs. FF +/− food BF infants less likely to STN at 26 weeks (χ	²	= 26.67,	p	< 0.0001) using 6 h criterion BF infants less likely to STN at 6 months (χ	²	= 31.19,	p	< 0.0001) using 8 h criterion	BF +/− food vs. FF +/− food BF infants less likely to STN at 52 weeks (χ	²	= 34.96,	p	< 0.0001) using 6 h criterion BF infants less likely to STN at 12 months (χ	²	= 25.24,	p	< 0.0001) using 8 h criterion	Chi-squared	4
DeLeon et al., 2007 ^[25]	DeLeon et al., 2007 [30]	39 weeks			BF +/− food vs. FF +/− food BF −ve correlated with total NTS (r = −0.42,	p	< 0.01)		Pearson’s correlation coefficient	4
Huang et al., 2016 ^[36]	Huang et al., 2016 [39]	0–34 weeks				BF +/− food vs. FF +/− food No assoc. between NTS %,	p	> 0.05	Multilevel mixed models	4
Mindell et al., 2012 [17]	13–52 weeks	⁴				BF +/− food vs. FF +/− food mean ± SD (h) 10.70 ± 1.03 vs. 10.30 ± 1.31,	p	= 0.146	MANCOVA	6
Night-Waking Frequency
Author, Year	Infant Age at Assessment/Assessment Frequency	≤6 Months		>6 Months	0–12 Months	Statistics ¹	Quality Rating ²
Author, Year	Infant Age at Assessment/Assessment Frequency	EBF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	Statistics ¹	Quality Rating ²
Berger et al., 2017 ^[32]	Berger et al., 2017 [37]	16 weeks		BF vs. FF No difference in no. of NW,	p	> 0.05			t	-test	6
Butte et al., 1992 ^[41]	Butte et al., 1992 [29]	17 weeks		BF vs. FF mean ± SD (no.) 2.9 ± 1.8 vs. 2.7 ± 2.0,	p	> 0.05			t	-test Regression	5
Figueiredo et al., 2017 ^[28]	Figueiredo et al., 2017 [38]	2, 13, 26 weeks	EBF vs. FF mean ± SD (no.) 2 weeks 3.02 ± 0.83 vs. 2.96 ± 0.88,	p	> 0.05 13 weeks 2.19 ± 1.07 vs. 1.65 ± 1.17,	p	> 0.05 26 weeks 2.22 ± 1.01 vs. 1.53 ± 0.90,	p	< 0.01	BF vs. FF mean ± SD (no.) 2 weeks 2.63 ± 0.67 vs. 2.96 ± 0.88,	p	> 0.05 13 weeks 2.18 ± 1.36 vs. 1.65 ± 1.17,	p	> 0.05 26 weeks 1.73 ± 0.94 vs. 1.53 ± 0.90,	p	> 0.05			Multivariate Analyses of Chi MANCOVA	³	5
Kaley et al., 2012 ^[26]	Kaley et al., 2012 [31]	4–10 weeks		BF vs. FF BF woke more freq. than FF,	p	< 0.05			Correlation ANOVA	4
Quillin et al., 1997 ^[34]	Quillin et al., 1997 [33]	4 weeks		BF vs. FF BF infants had more awakenings F(1,39) = 12.231,	p	< 0.01			ANOVA-two-way analysis of variance	3
Quillin et al., 2004 ^[35]	Quillin et al., 2004 [34]	4 weeks		BF vs. FF Mean ± SD (no.)	⁸	2.2 ± 0.8 vs. 2.0 ± 0.9,	p	> 0.05			t	-test	4
Rudzik et al., 2018 ^[38]	Rudzik et al., 2018 [43]	4, 6, 8, 10, 12, 14, 16, 18 weeks	EBF vs. FF Actigraphy report EBF has 2.1 less NW at 16 weeks,	p	= 0.05 No difference between grps for number of NW at 4, 6, 8, 10, 12, 14, 18 weeks				t	-test	3
Tikotzky et al., 2011 ^[39]	Tikotzky et al., 2011 [44]	26 weeks	EBF vs. FF EBF assoc. with more NW (Actigraph) (r = 0.32,	p	< 0.05)				Spearman rho correlations	6
Tikotzky et al., 2015 ^[37]	Tikotzky et al., 2015 [40]	26 weeks	EBF vs. FF Mean ± SD (no.)	⁹	2.53 ± 1.08 vs. 1.48 ± 0.96,	p	< 0.05				Spearman CC	5
Wailoo et al., 1990 ^[31]	Wailoo et al., 1990 [36]	13–17 weeks		BF vs. FF No difference in no. of NW,	p	> 0.05			t	-test	2
Brown et al., 2015 [19]	26–52 weeks			BF +/− food vs. FF +/− food No difference in total NW F(1711) = 0.931,	p	= 0.335		MANOVA	¹⁰	5
DeLeon et al., 2007 ^[25]	DeLeon et al., 2007 [30]	39 weeks			BF +/− food vs. FF +/− food BF +ve correlated with NW frequency (r = 0.48,	p	< 0.01)		Pearson’s correlation coefficient	4
Huang et al., 2016 ^[36]	Huang et al., 2016 [39]	0–34 weeks				BF +/− food vs. FF +/− food BF no diff as compared to FF for NW,	p	= 0.0700	Multilevel mixed models	4
Mindell et al., 2012 [17]	13–52 weeks	⁴				BF +/− food vs. FF +/− food mean ± SD (no.) 1.63 ± 1.24 vs. 0.94 ± 0.87,	p	= 0.003	MANCOVA	6
Sun et al., 2018 ^[27]	Sun et al., 2018 [35]	8–52 weeks				BF +/− food vs. FF +/− food Freq. NW assoc. with BF (v = 0.18,	p	= 0.002)	Chi-squared	t	-test
Duration of Night Wakings
Author, Year	Infant Age at Assessment/ Assessment Frequency	≤6 Months		>6 Months	0–12 Months	Statistics ¹	Quality Rating ²
Author, Year	Infant Age at Assessment/ Assessment Frequency	EBF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	Statistics ¹	Quality Rating ²
Figueiredo et al., 2017 ^[28]	Figueiredo et al., 2017 [38]	2, 13, 26 weeks	EBF vs. FF mean ± SD (h) 2 weeks 3.87 ± 1.13 vs. 4.38 ± 1.18,	p	> 0.05 13 weeks 3.03 ± 1.16 vs. 3.05 ± 1.20,	p	> 0.05 26 weeks 2.86 ± 1.01 vs. 2.87 ± 1.12,	p	> 0.05	BF vs. FF mean ± SD (h) 2 weeks 4.00 ± 1.11 vs. 4.38 ± 1.18,	p	> 0.05 13 weeks 3.00 ± 1.16 vs. 3.05 ± 1.20,	p	> 0.05 26 weeks 2.14 ± 0.90 vs. 2.87 ± 1.12,	p	> 0.05			Multivariate analyses of Chi MANCOVA	³	5
Yoshida et al., 2015 ^[33]	Yoshida et al., 2015 [45]	13, 17 weeks	EBF vs. FF EBF +ve correlated with wake time at night,	p	< 0.01				Multiple linear regression	5
DeLeon et al., 2007 ^[25]	DeLeon et al., 2007 [30]	39 weeks			BF +/− food vs. FF +/− food BF +ve correlated with duration of NW (r = 0.33,	p	< 0.05)		Pearson’s correlation coefficient	4
Longest Sleep Period
Author, Year	Infant Age at Assessment/ Assessment Frequency	≤6 Months		>6 Months	0–12 Months	Statistics ¹	Quality Rating ²
Author, Year	Infant Age at Assessment/ Assessment Frequency	EBF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	Statistics ¹	Quality Rating ²
Figueiredo et al., 2017 ^[28]	Figueiredo et al., 2017 [38]	2, 13, 26 weeks	EBF vs. FF mean ± SD (h) 2 weeks 3.04 ± 1.00 vs. 2.82 ± 0.90,	p	> 0.05 13 weeks 5.26 ± 2.15 vs. 6.50 ± 2.44,	p	< 0.05 26 weeks 5.38 ± 2.45 vs. 6.76 ± 1.96,	p	< 0.05	BF vs. FF mean ± SD (h) 2 weeks 3.38 ± 1.12 vs. 2.82 ± 0.90,	p	> 0.05 13 weeks 5.74 ± 2.31 vs. 6.50 ± 2.44,	p	> 0.05 26 weeks 6.98 ± 2.58 vs. 6.76 ± 1.96,	p	> 0.05			Multivariate analyses of Chi MANCOVA	³	5
Lee et al., 2000 ^[29]	Lee et al., 2000 [32]	2–17 weeks	EBF vs. FF mean ± SD (min)	¹¹	239.9 ± 102.7 vs. 274.1 ± 105.3,	p	< 0.01				Unpaired	t	-test	4
Rudzik et al., 2018 ^[38]	Rudzik et al., 2018 [43]	4, 6, 8, 10, 12, 14, 16, 18 weeks	EBF vs. FF Actigraph report EBF has 55 min-longer LSP at 18 weeks,	p	= 0.04 No difference between grps for LSP at 4, 6, 8, 10, 12, 14, 16 weeks				t	-test	3
Wailoo et al., 1990 ^[31]	Wailoo et al., 1990 [36]	13–17 weeks		BF vs. FF BF infants more likely to disturb parents within 4 h (χ	²	= 5.9, DF 3,	p	< 0.01)			t	-test	2
Huang et al., 2016 ^[36]	Huang et al., 2016 [39]	0–34 weeks				BF +/− food vs. FF +/− food No assoc. between LSP	p	> 0.05	Multilevel mixed models	4
Mindell et al., 2012 [17]	13–52 weeks	⁴				BF +/− food vs. FF +/− food mean ± SD (h) 7.06 ± 2.73 vs. 7.85 ± 2.75,	p	= 0.249	MANCOVA	6
Sleep Onset Latency
Author, Year	Infant Age at Assessment/Assessment Frequency	≤6 Months		>6 Months	0–12 Months	Statistics ¹	Quality Rating ²
Author, Year	Infant Age at Assessment/Assessment Frequency	EBF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	BF vs. FF ¹	Statistics ¹	Quality Rating ²
Butte et al., 1992 ^[41]	Butte et al., 1992 [29]	17 weeks		BF vs. FF EEG: mean ± SD (min) 34.3 ± 41.6 vs. 4.0 ± 12.6,	p	< 0.05			t	-test Regression	5
Figueiredo et al., 2017 ^[28]	Figueiredo et al., 2017 [38]	2, 13, 26 weeks	EBF vs. FF mean ± SD (h) 2 weeks 0.33 ± 0.31 vs. 0.48 ± 0.40,	p	> 0.05 13 weeks 0.42 ± 0.45 vs. 0.42 ± 0.52,	p	> 0.05 26 weeks 0.39 ± 0.35 vs. 0.57 ± 0.72,	p	> 0.05	BF vs. FF mean ± SD (h) 2 weeks 0.56 ± 0.75 vs. 0.48 ± 0.40,	p	> 0.05 13 weeks 0.44 ± 0.41 vs. 0.42 ± 0.52,	p	> 0.05 26 weeks 0.51 ± 0.31 vs. 0.57 ± 0.72,	p	> 0.05			Multivariate analyses of Chi MANCOVA	³	5
Tikotzky et al., 2011 ^[39]	Tikotzky et al., 2011 [44]	26 weeks	EBF vs. FF EBF assoc. with later sleep onset (r = 0.32,	p	< 0.05)				Spearman rho correlations	6
Mindell et al., 2012 [17]	13–52 weeks	⁴				BF +/− food vs. FF +/− food mean ± SD (h) 0.23 ± 0.15 vs. 0.30 ± 0.53,	p	= 0.427	MANCOVA	6

2.1.1. 24 h Sleep Duration

Among infants aged 6 months and younger, exclusive breastfeeding was not associated with 24 h sleep duration in one study ^[39][44] and no difference in sleep duration compared to formula-feeding was reported in a cohort study ^[28][38]. However, one study reported significantly longer sleep duration in exclusively breastfed infants compared to formula-fed infants ^[29][32]. There was no reported difference in sleep duration of breastfed infants compared to formula-fed infants in four studies ^{[23][41][26][28]}[23,29,31,38]. In contrast, one study found that breastfed infants had significantly shorter sleep duration ^[35][34], while another study reported a significantly longer sleep duration compared to formula-fed infants ^[32][37].

Among infants older than 6 months, 24 h sleep duration did not differ between breastfed and formula-fed infants in one study [23], whilst two other studies reported significantly shorter sleep duration in breastfed infants compared to formula-fed infants ^[42][30][42,46].

Breastfeeding was associated with significantly shorter sleep duration compared to formula feeding in a study that examined infants 0 to 8 months-of-age ^[36][39].

2.1.2. Total Night-Time Sleep

Among infants aged 6 months and younger, no difference in total night-time sleep was reported between exclusively breastfed and formula-fed infants in one cohort and one longitudinal study, where sleep was measured at multiple time points ^[28][38][38,43]. However, one study reported exclusively breastfed infants at 3 and 4 months-of-age were less likely to sleep through the night compared to formula-fed infants ^[33][45]. There was no reported difference in total night-time sleep in breastfed infants compared to formula-fed infants in three studies ^[26][35][28][31,34,38]. However, three other studies reported a significantly shorter night-time sleep duration experienced by breastfed infants ^[41][34][40][29,33,41], while one study reported breastfed infants to have a significantly longer night-time sleep duration compared to formula-fed infants ^[32][37].

Among infants older than 6 months, breastfeeding was inversely associated with night-time sleep duration in two studies ^[25][40][30,41].

No difference in night-time sleep duration was found in breastfed infants compared to formula-fed infants aged 3 to 12 months [17] and 0 to 8 months ^[36][39].

2.1.3. Night-Waking Frequency

In two studies, there was no reported differences in the frequency of night wakings in infants aged 6 months and younger who were exclusively breastfed or formula-fed ^[28][38][38,43]. However, one of these studies reported that infants that exclusively breastfed had significantly fewer night wakings at 16 weeks compared to formula-fed infants ^[38][43]. The majority of studies (n = 3) reported that exclusively breastfed infants had a significantly higher number of night-time wakings compared to formula-fed infants ^[28][37][39][38,40,44]. In contrast, the majority of the studies (n = 5) of breastfed infants and formula-fed infants reported no association ^{[41][35][31][32][28]}[29,34,36,37,38], with two studies reporting that breastfed infants had significantly more night-time wakings than formula-fed infants ^[26][34][31,33].

No association between night-waking frequency and breastfeeding compared to formula-fed infants older than six months was reported in one study [19]. Another study found breastfed infants to have a significantly greater frequency of night awakening compared to formula-fed infants ^[25][30].

In one study, no differences were found in night-waking frequency in infants aged 0–8 months who were breastfed or formula-fed ^[36][39]. However, breastfed infants had a significantly greater number of night-time wakings than formula-fed infants aged 3 to 12 months [17] and 2 to 12 months ^[27][35].

2.1.4. Duration of Night Wakings

Among infants aged 6 months and younger, no difference was reported in duration of night wakings between exclusively breastfed and formula-fed infants in one study ^[28][38]. However, another study reported a significantly longer duration of night wakings in exclusively breastfed infants compared to formula-fed infants ^[33][45].

Among infants older than 6 months, breastfeeding was reported to result in a significantly longer duration of night wakings compared to formula-fed infants in one study ^[25][30].

2.1.5. Longest Sleep Period

Two studies reported no difference in longest sleep period between exclusive breastfeeding and formula feeding among infants aged 6 months and younger ^[28][38][38,43]. In contrast, two studies ^[29][28][32,38] found exclusively breastfed infants to have a significantly shorter longest sleep period compared to infants who were formula-fed. Of those studies, two studies measured the longest sleep period at multiple time points, one cohort ^[28][38] and another longitudinal design ^[38][43]. The longitudinal study ^[38][43] also found exclusively breastfed infants at 18 weeks-of-age to have a significantly longer longest sleep period as compared to formula-fed infants. Breastfed infants, when compared to formula-fed infants, reported no difference in longest sleep period in one study ^[28][38], while another study reported breastfed infants were more likely to wake parents in a four hour period than formula-fed infants ^[31][36].

No difference in longest sleep period duration was found in breastfed infants compared to formula-fed infants aged 3 to 12 months [17] and 0 to 8 months ^[36][39].

2.1.6. Sleep Onset Latency

Among infants aged 6 months and younger, one study reported no difference in sleep onset latency between exclusively breastfed and formula-fed infants ^[28][38], while another study reported exclusive breastfeeding was associated with later sleep onset time ^[39][44]. When breastfed infants were compared to formula-fed infants, no difference in sleep onset latency was reported in one study ^[28][38], while another study reported longer sleep onset duration associated with breastfed infants ^[41][29].

No difference in sleep onset latency was found in breastfed infants when compared to formula-fed infants aged 3 to 12 months [17].

2.2. Introduction to Complementary Foods and Infant Sleep

Four studies (two cohort, one combined RCT, and one cross-sectional) examined the association between the timing of introduction to complementary foods and infant sleep. All sleep measurements were subjective, and information on timing of introduction to complementary foods was retrospectively collected from parents or caregivers.

2.2.1. 24 h Sleep Duration

Three studies reported the relationship between the timing of introduction to complementary foods and 24 h sleep duration. No difference was reported in 24 h sleep duration assessed at six [23] and nine ^[42][30][42,46] months-of-age among infants introduced to complementary foods at ≤12 weeks (around 3 months) compared to at >12 weeks-of-age, at <4 months compared to at ≥4 months-of-age, and at <26 weeks (at 6 months) compared to at ≥26 weeks-of-age. However, infants slept 24 min less (−0.39, 95% CI: −0.67–−0.11) at 12 months-of-age if they were introduced to complementary foods at <4 months compared to at ≥4 months-of-age [23].

Only two out of the four studies ^[42][30][42,46], examined the relationship between the timing of introduction to complementary foods and sleep among breastfed infants separately from formula-fed infants. Heinig et al. [42] reported similar results in both breastfed and formula-fed infants, where no difference in 24 h sleep duration was reported in relation to the timing of introduction to complementary foods. However, Morgan et al. ^[30][46] found that breastfed infants were more likely to sleep through the night at 9 months with early introduction of complementary foods at ≤12 weeks as compared to at >12 weeks. This was not observed in formula-fed infants. However, the authors were unable to further their investigation due to the lack of data collected on the reasons for infants waking (i.e., waking for feeding or waking and self-soothing to sleep).

2.2.2. Night-Waking Frequency

One study of infants aged 6–12 months reported no significant association with night-waking frequency and timing of introduction to complementary foods (mean age of introduction 21 weeks); however, an association between later introduction to complementary foods and number of feeds during the night was reported (independent of infant age) [19].

2.3. Quality Assessment

21 studies were included in the quality assessment. Quality scores ranged from 2 to 6 points (out of a total of 6 points). The mean quality score was 4. The quality assessment variable with the lowest score was the additional question on the sleep assessment tool validation, where only 10 out of 21 studies reported using validated sleep assessment methods ^{[17][41][34][35][27][32][37][38][39][33]}[17,29,33,34,35,37,40,43,44,45]. Four (19%) studies ^{[17][27][32][39]}[17,35,37,44] had a quality score of 6, whilst 14 (66%) ^{[19][23][41][25][26][29][35][28][36][37][40][42][33][30]}[19,23,29,30,31,32,34,38,39,40,41,42,45,46] scored > 4 points, and 3 studies (14%) ^[34][31][38][33,36,43] scored ≤ 3 points. The three studies had low rating (≤3 points) as their participants were not representative of the target population ^[34][31][38][33,36,43], had inappropriate outcome measurements ^[31][36], had less than 90% completed data collection ^[34][38][33,43], did not account for confounders ^[34][33], the feeding group was not administered as intended ^[38][43], and did not use a validated sleep assessment tool ^[31][36].

3. Conclusions

Exclusively breastfed infants (≤6 months-of-age) were reported more likely to wake at night compared to formula-fed infants, though this association was not found in breastfed infants (partial or predominantly breastfed). The majority of the studies reported no difference in night-time sleep duration and total 24 h sleep duration in both exclusively breastfed and breastfed infants (≤6 months-of-age) compared to formula-fed infants. However, after 6 months-of-age, most studies reported breastfed infants to sleep less than formula-fed infants. Though studies were limited, the majority observed no association on the timing of introduction to complementary foods and total 24 h sleep duration, including one study that compared infants who were introduced complementary foods before and after 6 months-of-age in accordance with the WHO recommendations.