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.
