The cortisol awakening response (CAR) in toddlers: Nap-dependent effects on the diurnal secretory pattern

Abstract

INTRODUCTION Cortisol levels in adults show a sharp decrease from midmorning to midafternoon. Most toddlers take afternoon naps, which is associated with a less mature diurnal pattern characterized by a midday plateau in cortisol secretion. Napping in preschoolers produces a robust cortisol awakening response (CAR), which may account for such maturational differences. This experimental study extends prior work by examining whether the presence and timing of the nap-dependent CAR influences the diurnal cortisol pattern in toddlers. METHODS Toddlers (n=28; 13 females; 30–36 months) followed a strict biphasic sleep schedule (≥12.5h time in bed; ≥90 min nap) for ≥3 days before each of four randomly ordered, in-home cortisol assessments. For each assessment, saliva samples were obtained at morning awakening, ~09:30, pre-nap, 0, 15, 30, 45, 90, 135 min post-nap awakening (verified with actigraphy), and ~19:30. On one day, children napped at their scheduled time, and parents collected saliva samples. On another day, children missed their nap, and parents collected saliva samples at matched times. On two other days, children napped 4 h (morning) and 7 h (afternoon) after awakening in the morning, during which time researchers collected pre- and post-nap saliva samples. Saliva was assayed for cortisol (μg/dl). RESULTS Three-level multilevel models were used to estimate the CAR and diurnal cortisol patterns in all four conditions. Compared to the no-nap condition (no observed CAR; b= −0.78, p=0.65), we found a pronounced cortisol rise following the morning nap (b=11.00, p<0.001) and both afternoon naps whether samples were collected by parents (b=5.19, p<0.01) or experimenters (b= 4.97, p<0.01). Napping in the morning resulted in the most robust post-nap cortisol rise (b=10.21, p<0.001). Diurnal patterns were analyzed using piecewise growth modeling that estimated linear coefficients for five separate periods throughout the day (corresponding to morning decline, noon decline, post-nap rise, post-nap decline, and evening decline). We observed a significant post-nap rise in cortisol values on the parent-collected afternoon nap (b=3.41, p<0.01) and the experimenter-collected morning nap (b=7.50, p<0.01) days as compared to the no-nap day (b=−0.17, p=0.82). No other differences in diurnal profiles were observed between the parent-collected nap and no-nap conditions; however, toddlers had a steeper evening decline on the day of the morning nap compared to the parent-collected afternoon nap (b=0.30, p<0.05) and no-nap conditions (b=0.27, p<0.05). DISCUSSION These well-controlled findings suggest that the presence and timing of daytime naps influence the pattern of diurnal cortisol secretion in toddlers. They also provide support for the hypothesis that napping is the primary state driving the immature midday plateau in cortisol secretion, which becomes more adult-like across childhood. Prior studies of the diurnal cortisol pattern have employed a cubic model, and therefore, have not detected all possible variations due t...

Toddlers’ diurnal cortisol levels affected by out-of-home, center-based childcare and at-home, guardian-supervised childcare: comparison between different caregiving contexts

“…These cortisol increases were partly explained by the afternoon naps, as about 35% of the afternoon saliva samples were taken 15-60 min after waking up. This association between afternoon napping and the cortisol levels was expected, as napping has been found to result in a post-nap cortisol rise in toddlers [21]. Previous research studies have defined the cortisol awakening response (CAR), which describes the period of increased cortisol secretion approximately 30-45 min post-awakening [20].…”

“…Thus, cortisol secretion does not only follow the circadian rhythm but is also nap-dependent in toddlers. The pronounced cortisol rise is observed to follow both the morning and afternoon naps in comparison with the no-nap condition [21].…”

“…Previous research studies have defined the cortisol awakening response (CAR), which describes the period of increased cortisol secretion approximately 30-45 min post-awakening [20]. Most toddlers take naps, and an increase in cortisol values have also been found following both morning and afternoon naps [21].…”

Developmental effects on sleep–wake patterns in infants receiving a cow’s milk-based infant formula with an added prebiotic blend: a Randomized Controlled Trial

“…By assessing nighttime along with daytime sleep–wake patterns, wake time and daytime napping have been demonstrated to induce modifications of CAR. 40 , 41 Previous studies have suggested that emergence of CAR varies by individual and can appear from 2 to 6 months of age. 10 , 11 Current study results confirm the presence of CAR by ~2 months of age.…”

Toddlers’ stress during transition to childcare

“…It is also possible that some younger children have a nap after childcare, resulting in an elevation of cortisol, which is called a cortisol awakening response (CAR) (Tervahartiala et al 2019). However, evening napping has been observed to not produce much cortisol (Tribble et al 2015). To alleviate tension, children might benefit from calm evenings with focused care from parents (Ahnert and Lamb 2003).…”