Time of night and first night effects on arousal response in healthy adults.

OBJECTIVE Several factors, such as homeostatic and circadian influences, may affect the density of cortical and subcortical arousals (AR). The purpose of this study was to examine the time-of-night and the first night effect on AR response. METHODS AR were classified into microarousals (MA), phases of transitory activation (PAT), delta (D-burst) and K-complex burst (K-burst). The AR density and duration was analyzed during two consecutive nights with the analysis of sleep stage and sleep cycle in thirty-six healthy subjects. RESULTS D- and K-burst showed a trend toward progressive decline across sleep cycles (p<0.0001). While MA rate was unaffected throughout sleep cycles, PAT index increased across the night (p=0.002). The density and duration of each group of AR exhibited reproducibility without significant differences between nights. An individual inter-night variability in AR density was found independently of night and sleep structure. CONCLUSIONS While homeostatic and circadian influences affect nighttime subcortical and MA responses, a wakefulness drive modulates the occurrence of AR with movements. Although the pattern of AR responses was highly reliable from the first to second night, the substantial inter-individual variability suggests the existence of an individual susceptibility. SIGNIFICANCE The first night effect on arousal response is affected by individual susceptibility and circadian and homeostatic influences.