Descriptive statistics (Mean ± SD) for circadian and actigraphic sleep measures for napping (n = 15) and non-napping (n = 5) toddlers.

<p>Statistics are shown for independent t-tests (napping versus non-napping participants).</p><p>^§ indicates a two-tailed test.</p><p>Descriptive statistics (Mean ± SD) for circadian and actigraphic sleep measures for napping (n = 15) and non-napping (n = 5) toddlers.</p

The Timing of the Circadian Clock and Sleep Differ between Napping and Non-Napping Toddlers

<div><p>The timing of the internal circadian clock shows large inter-individual variability across the lifespan. Although the sleep-wakefulness pattern of most toddlers includes an afternoon nap, the association between napping and circadian phase in early childhood remains unexplored. This study examined differences in circadian phase and sleep between napping and non-napping toddlers. Data were collected on 20 toddlers (34.2±2.0 months; 12 females; 15 nappers). Children followed their habitual napping and non-napping sleep schedules (monitored with actigraphy) for 5 days before an in-home salivary dim light melatonin onset (DLMO) assessment. On average, napping children fell asleep during their nap opportunities on 3.6±1.2 of the 5 days before the DLMO assessment. For these napping children, melatonin onset time was 38 min later (<i>p</i> = 0.044; <i>d</i> = 0.93), actigraphically-estimated bedtime was 43 min later (<i>p</i> = 0.014; <i>d</i> = 1.24), sleep onset time was 59 min later (<i>p</i> = 0.006; <i>d</i> = 1.46), and sleep onset latency was 16 min longer (<i>p</i> = 0.030; <i>d</i> = 1.03) than those not napping. Midsleep and wake time did not differ by napping status. No difference was observed in the bedtime, sleep onset, or midsleep phase relationships with DLMO; however, the wake time phase difference was 47 min smaller for napping toddlers (<i>p</i> = 0.029; <i>d</i> = 1.23). On average, nappers had 69 min shorter nighttime sleep durations (<i>p</i> = 0.006; <i>d</i> = 1.47) and spent 49 min less time in bed (<i>p</i> = 0.019; <i>d</i> = 1.16) than non-nappers. Number of days napping was correlated with melatonin onset time (<i>r</i> = 0.49; <i>p</i> = 0.014). Our findings indicate that napping influences individual variability in melatonin onset time in early childhood. The delayed bedtimes of napping toddlers likely permits light exposure later in the evening, thereby delaying the timing of the clock and sleep. Whether the early developmental trajectory of circadian phase involves an advance associated with the decline in napping is a question necessitating longitudinal data as children transition from a biphasic to monophasic sleep-wakefulness pattern.</p></div

Demographics by individual at each age in the younger and older cohorts.

a<p>Participants transitioned from Tanner 1 to 2 at 10 years (n = 5), 11 years (n = 6), and 12 years (n = 5).</p>b<p>Participants transitioned from Tanner 1 to 3 at 11 years (n = 1).</p>c<p>Participants transitioned from Tanner 2 to 3 at 12 years (n = 1) and 13 years (n = 1).</p>d<p>Participants transitioned from Tanner 3 to 4 at 11 years (n = 2), 13 years (n = 1), and 15 years (n = 1).</p>e<p>Participants transitioned from Tanner 3 to 5 at 11 years (n = 2).</p>f<p>Participants transitioned from Tanner 4 to 5 at 15 years (n = 3) and 16 years (n = 1).</p><p>Notes: if more than one Morningness/Eveningness score was collected at each age, then the mean score was used; Tanner stage was unavailable for 1 participant at ages 9, 11, and 13 years, and for 2 participants at age 15 years.</p><p>Demographics by individual at each age in the younger and older cohorts.</p

Scatterplot showing the association between napping and circadian phase in toddlers (n = 20).

<p>Number of days that toddlers napped during the 5-days preceding the dim light melatonin onset (DLMO) assessment was estimated with actigraphy (<i>r</i> = 0.49, <i>p</i> = 0.014).</p

Means (SDs) for actigraphic sleep and circadian outcomes by age in the younger and older cohorts.

<p>Notes: These data are from 94 participants (N = 38 in the younger cohort; N = 56 in the older cohort) who contributed on average 4.29 assessments range (1 to 6). Three observations at age 19 were included in the 18+ category.</p><p>Means (SDs) for actigraphic sleep and circadian outcomes by age in the younger and older cohorts.</p

Modeled developmental trajectories (bold line) and individual trajectories (thin lines) for DLMO phase (A), DLMO phase angle to sleep onset (B), and DLMO phase angle to sleep offset (C).

<p>A negative DLMO phase angle to sleep onset indicates when the DLMO occurred after weekday sleep onset.</p

Modeled developmental trajectories (bold line) and individual trajectories (thin lines) for actigraphically estimated sleep onset and offset on weekdays (A and D) and weekends (B and E) in the proximal 7 days before DLMO phase was measured in both cohorts.

<p>Sleep onset and offset differences between weekends and weekdays (C and F) illustrate when participants slept earlier (<0) or later (>0) on weekends compared to weekdays. The younger cohort (9–13 years) is on the left and the older cohort (15–19 years) is on the right of each plot.</p

Predefined brain regions, for which VF_M was measured using the mcDESPOT [multicomponent Driven Equilibrium Single Pulse Observation of T₁ and T₂], white matter imaging technique.

<p>Predefined brain regions, for which VF_M was measured using the mcDESPOT [multicomponent Driven Equilibrium Single Pulse Observation of T₁ and T₂], white matter imaging technique.</p

Significant Pearson correlations between inspection times [ln ms] and myelin volume fraction (VF_M) in the left occipital lobe, left and right internal capsules, and right superior longitudinal fasciculus (SLF), before controlling for age.

<p>Significant Pearson correlations between inspection times [ln ms] and myelin volume fraction (VF_M) in the left occipital lobe, left and right internal capsules, and right superior longitudinal fasciculus (SLF), before controlling for age.</p

Descriptive statistics for age, inspection times and myelin volume fraction (VF_M), raw Pearson correlations with age or inspection times, and partial correlations with inspection times (controlling for age).

<p>VF_M = myelin volume fraction. M = Mean. SD = Standard Deviation. L. = Left. R. = Right. SLF = Superior longitudinal fasciculus. CC = Corpus callosum. Significant correlations appear in bold.</p><p>Descriptive statistics for age, inspection times and myelin volume fraction (VF_M), raw Pearson correlations with age or inspection times, and partial correlations with inspection times (controlling for age).</p