Is cardiovascular autonomic control during the day-to-night transition dependent on dim light melatonin onset? An exploratory analysis in healthy adults.
Brito L, Jones M, et al. • Physiological reports • 2026
CV autonomic transition into biological night was not associated with DLMO, which seems to depend on the phase angle of entrainment.
Key Findings
Results
Cardiovascular autonomic onset times for all indices did not differ significantly from DLMO.
Study included 10 healthy adults (4 females, age 25 ± SD 2 years) under an ultra-short semi-constant routine protocol (<8 lux)
Participants remained seated for 7 hours until 1 hour after their habitual bedtime
Statistical comparison yielded p > 0.05 for all cardiovascular autonomic indices versus DLMO
CV onset time was identified using a segmental (piecewise) analysis to find the inflection point (X0 slope) throughout the study
Results
Mean differences between DLMO and CV onset time were significantly correlated with the phase angle of entrainment.
All correlations between DLMO-CV onset time differences and phase angle were significant with r > +0.75
Phase angle was defined as the time difference between DLMO and the midpoint of self-reported sleep
Phase angle served as a measure of alignment between sleep-wake patterns and biological circadian rhythms
This finding held across all cardiovascular autonomic indices examined
Methods
DLMO was defined as the interpolated time for melatonin increase over 3 pg/mL using saliva samples collected every 30 minutes.
Saliva samples were collected every 30 minutes throughout the protocol
The 3 pg/mL threshold was used as the standard for defining DLMO
R-R intervals and beat-to-beat blood pressure were recorded every hour
CV autonomic modulation was assessed using indices of heart rate and blood pressure variability
Discussion
The relationship between the cardiovascular autonomic day-to-night transition and DLMO appears to depend on the phase angle of entrainment rather than DLMO itself.
Individuals with different phase angles showed different temporal relationships between DLMO and CV autonomic onset
The exploratory analysis suggests that sleep-wake timing relative to internal circadian phase (phase angle) modulates how closely CV autonomic changes track melatonin onset
The study was conducted in healthy young adults (mean age 25 years), limiting generalizability
The authors characterize this as an exploratory analysis
What This Means
This research suggests that the body's internal clock influences the heart and blood vessels in complex ways that are not simply tied to the rise of melatonin at night. The study measured melatonin levels and cardiovascular measures (heart rate variability and blood pressure variability) in 10 healthy young adults kept in dim light for 7 hours. The researchers looked for whether a key circadian marker—dim light melatonin onset (DLMO), the point when the body starts producing melatonin in the evening—lined up with the moment when the cardiovascular system shifted from its daytime to nighttime pattern of activity.
While the timing of cardiovascular changes was statistically similar to DLMO on average, the degree of mismatch between the two was strongly linked to each person's 'phase angle of entrainment'—essentially how closely their sleep timing was aligned with their internal biological clock. People whose sleep schedules were well-aligned with their melatonin rhythms showed a tighter correspondence between melatonin onset and cardiovascular changes, while those whose schedules were less aligned showed a greater mismatch.
This research suggests that the cardiovascular system's day-to-night transition is not simply driven by melatonin onset, but is influenced by the broader relationship between a person's sleep habits and their internal circadian timing. This could be relevant to understanding why people with misaligned sleep schedules (such as shift workers or those with social jetlag) may face different cardiovascular risks, though the small sample size (10 participants) means these findings should be considered preliminary and exploratory.
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Brito L, Jones M, Garcês C, Shea S, Thosar S. (2026). Is cardiovascular autonomic control during the day-to-night transition dependent on dim light melatonin onset? An exploratory analysis in healthy adults.. Physiological reports. https://doi.org/10.14814/phy2.71145