The Timing of the Circadian Clock and Sleep Differ between Napping and Non-Napping Toddlers.
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 (p = 0.044; d = 0.93), actigraphically-estimated bedtime was 43 min later (p = 0.014; d = 1.24), sleep onset time was 59 min later (p = 0.006; d = 1.46), and sleep onset latency was 16 min longer (p = 0.030; d = 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 (p = 0.029; d = 1.23). On average, nappers had 69 min shorter nighttime sleep durations (p = 0.006; d = 1.47) and spent 49 min less time in bed (p = 0.019; d = 1.16) than non-nappers. Number of days napping was correlated with melatonin onset time (r = 0.49; p = 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.
Sleep and Development Laboratory, Department of Integrative Physiology, University of Colorado at Boulder, Boulder, CO, United States of America.
Lameese D
LD
Akacem
Sleep and Development Laboratory, Department of Integrative Physiology, University of Colorado at Boulder, Boulder, CO, United States of America; College of Osteopathic Medicine, Rocky Vista University, Parker, CO, United States of America.
Charles T
CT
Simpkin
Sleep and Chronobiology Laboratory, EP Bradley Hospital, Department of Psychiatry and Human Behavior, The Warren Alpert Medical School of Brown University, Providence, RI, United States of America; Centre for Sleep Research, School of Psychology, Social Work and Social Policy, University of South Australia, Adelaide, Australia.
Mary A
MA
Carskadon
Sleep and Chronobiology Laboratory, Department of Integrative Physiology, University of Colorado at Boulder, Boulder, CO, United States of America.
Kenneth P
KP
Wright
Child Development Center, University Children's Hospital Zurich, Zurich, Switzerland.
Oskar G
OG
Jenni
Institute of Pharmacology and Toxicology, Section of Chronobiology and Sleep Research, University of Zurich, Zurich, Switzerland.
Peter
P
Achermann
Sleep and Development Laboratory, Department of Integrative Physiology, University of Colorado at Boulder, Boulder, CO, United States of America.
Monique K
MK
LeBourgeois
D056044:Actigraphy / Q000379:methods; D002675:Child, Preschool; D057906:Circadian Clocks; D002940:Circadian Rhythm; D005260:Female; D006801:Humans; D008297:Male; D008550:Melatonin / Q000032:analysis*; D012463:Saliva / Q000737:chemistry; D012890:Sleep / Q000502:physiology*; D013997:Time Factors
D008550:Melatonin