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.