Is cardiovascular autonomic control during the day-to-night transition dependent on dim light melatonin onset? An...

We tested whether dim-light melatonin onset (DLMO) is time-aligned with the transition between biological day into night in cardiovascular autonomic control (CV onset time). We studied 10 healthy adults (4 females, age 25 ± SD 2 years) under an ultra-short semi-constant routine protocol (<8lux). Participants remained seated for 7 h until 1 h after their habitual bedtime. We collected saliva samples every 30 min and recorded R-R intervals and beat-to-beat blood pressure every hour. DLMO was defined as the interpolated time for melatonin increase over 3 pg/mL. Phase angle was defined as the time difference between DLMO and the midpoint of self-reported sleep as a measure of the alignment between sleep-wake patterns and biological circadian rhythms. CV onset time was defined using a segmental (piecewise) analysis to identify the inflection point (X₀ slope) throughout the study. CV autonomic modulation was assessed using indices of heart rate and blood pressure variability. CV onset times for all indices did not differ significantly from DLMO (p > 0.05). Mean differences between DLMO and CV onset time were significantly correlated with the phase angle of entrainment (all significant, r > +0.75). Our results suggest that CV autonomic transition into biological night was not associated with DLMO, which seems to depend on the phase angle of entrainment.