The study, conducted at Oregon Health & Science University, set out to test a long-standing hypothesis. Resting cardiac parasympathetic activity is known to increase before the biological night, while norepinephrine, a marker of sympathetic nervous system activity, peaks in the late biological afternoon and falls to its lowest levels during the night. Because melatonin normally begins rising about two hours before habitual sleep in healthy young adults, scientists had proposed that this hormonal surge might orchestrate the transition of cardiovascular control into its nighttime resting mode. That idea had already run into trouble: earlier experiments that manipulated melatonin levels around bedtime, either by administering the hormone or by suppressing it with bright light of more than 3000 lux, found no accompanying change in cardiac autonomic modulation. What remained unknown was whether the natural, endogenous increase in melatonin, marked by the dim light melatonin onset, or DLMO, coincides with and possibly facilitates the autonomic transition.

To find out, the researchers recruited ten healthy adults aged 20 to 39 years, screened to exclude chronic medical conditions, prescribed medications, recent night-shift work, tobacco, marijuana and illicit drug use, and pregnancy. Female participants were studied during the early follicular phase of the menstrual cycle, with two exceptions using hormonal birth control. Eligibility required resting blood pressure below 130 mmHg systolic and 80 mmHg diastolic, and a body mass index between 18.5 and 29.99 kilograms per square meter. For seven days before the laboratory visit, participants followed a self-selected eight-hour in-bed schedule, confirmed daily through time-stamped phone calls and sleep diaries. Adherence was striking: bedtime variability averaged just six minutes and sleep midpoint variability five minutes, a regularity that matters enormously when the goal is to measure the body’s internal timing with precision.

On the day of the visit, participants arrived eight hours before their chosen bedtime and remained in the laboratory until one hour after it. After eligibility checks, lights were dimmed to below eight lux, and participants stayed seated in a comfortable chair for seven hours in what the team calls an ultra-short semi-constant routine protocol. This design controls for the major confounders of autonomic measurements, namely body posture, sleep and light exposure, while still allowing water and light snacks. Every thirty minutes, researchers collected saliva samples to track melatonin; every hour, they recorded ten minutes of continuous electrocardiogram and beat-by-beat blood pressure using a finger-cuff photoplethysmography system validated against intra-arterial measurements. DLMO was defined as the interpolated time at which salivary melatonin exceeded three picograms per milliliter, and it averaged 8:26 p.m. across the group.

From the cardiovascular recordings, the team derived several indices of autonomic control. The root mean square of successive differences between adjacent normal heartbeats, or RMSSD, served as the primary marker of cardiac parasympathetic modulation. The ratio of low-frequency to high-frequency power in heart rate variability provided an estimate of cardiac sympathovagal balance, while the low-frequency band of systolic blood pressure variability indexed sympathetic vasomotor modulation. The key methodological innovation lay in determining when each of these variables changed during the evening. Using segmental linear regression, a piecewise model that identifies the inflection point where a variable’s slope shifts, the researchers pinpointed each participant’s individual onset time for every cardiovascular index. All fitted models showed goodness-of-fit values of at least 0.6, and an experienced researcher verified that each identified point reflected a genuine physiological transition rather than transient noise.

The central result was a null finding with an intriguing twist. When the onset times of the cardiovascular variables were compared with DLMO, none of the differences reached statistical significance. The mean difference was 34 minutes for the R-R interval, minus 25 minutes for RMSSD, minus 33 minutes for the low-to-high-frequency ratio, 18 minutes for systolic blood pressure, and 12 minutes for low-frequency systolic blood pressure variability, all with p-values well above 0.05 and small to moderate effect sizes ranging from 0.14 to 0.39. In other words, the autonomic transition sometimes preceded the melatonin rise and sometimes followed it, varying by index and by individual. The endogenous increase in melatonin, at least as marked by DLMO, does not appear to determine when the cardiovascular system shifts into its biological-night mode.

Yet the transition clearly clustered around DLMO, and the individual variability in timing proved to be the study’s most striking discovery. The researchers calculated each participant’s phase angle of entrainment, the time difference between DLMO and the average sleep midpoint reported in the seven-day diaries. This measure of circadian alignment correlated strongly and positively with the timing gap between DLMO and the onset of every cardiovascular variable except the R-R interval, with correlation coefficients between 0.76 and 0.89 and adjusted p-values below 0.03 after Benjamini-Hochberg correction for multiple comparisons. Participants with a wider phase angle, meaning a longer interval between melatonin onset and sleep midpoint, showed their cardiovascular transition after DLMO, while those with a narrower phase angle showed it before. Notably, diary-reported bedtime and sleep midpoint were largely unrelated to these timing differences, with the exception of the R-R interval, suggesting that the phase angle itself, rather than sleep timing alone, captures the relevant physiology.

The authors interpret this pattern with appropriate caution, emphasizing the observational nature of the study and its hypothesis-generating intent. They propose that people whose circadian system is more tightly aligned with their sleep schedule may exhibit a kind of physiological readiness, anticipating the transition to biological night rather than merely reacting to environmental or behavioral changes through negative feedback. Supporting evidence for a link between circadian alignment and autonomic health comes from other work: ambulatory data from 360 adolescents showed that later midpoint sleep was associated with increased cardiac sympathovagal balance both awake and asleep, and adults with greater social jet lag, the mismatch in sleep timing between workdays and free days, showed reduced cardiac parasympathetic modulation during the first three hours of sleep. Controlled laboratory studies reinforce the point, with experimentally induced circadian misalignment raising wake systolic and 24-hour average diastolic blood pressure after a twelve-hour inversion of the sleep-wake cycle.

The study carries important limitations that the authors acknowledge candidly. With only ten participants, it was not powered for these particular questions, and the sample cannot detect sex differences in DLMO timing, which previous research suggests may occur earlier in females. Melatonin was not sampled throughout the night, restricting conclusions to the day-to-night transition, and cardiovascular autonomic data were collected only hourly, which may limit the precision of the estimated onset times. The findings also apply only to young, ostensibly healthy adults and should not be generalized to older people or those with hypertension, obesity or other cardiovascular risk factors, all of which can affect autonomic modulation. The authors call for larger studies with more frequent autonomic sampling and continuous melatonin measurement, as well as experimental protocols that impose circadian misalignment to test whether alignment causally shapes the efficiency of physiological transitions.

For now, the message is one of refined understanding rather than clinical application. The nightly handover of cardiovascular control appears to be governed by the endogenous circadian system in concert with behavioral alignment, not by the melatonin signal alone. That the transition occurs near DLMO in everyone, yet before or after it depending on an individual’s phase angle, reframes the question from whether melatonin triggers the shift to how circadian alignment shapes the body’s preparedness for the biological night. In an era when irregular sleep schedules and light exposure at night are increasingly common, understanding how the circadian system times this transition, and what happens when that timing goes awry, may prove essential to explaining why circadian misalignment carries measurable cardiovascular costs.

Subject of Research: Circadian timing of cardiovascular autonomic control and its relation to dim light melatonin onset in healthy adults

Article Title: Is cardiovascular autonomic control during the day‐to‐night transition dependent on dim light melatonin onset? An exploratory analysis in healthy adults

Article References: Brito, L. C., Jones, M., Garcês, C. P., Shea, S. A., & Thosar, S. 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, 14(20), Article e71145. https://doi.org/10.14814/phy2.71145

Image Credits: AI Generated

DOI: 10.14814/phy2.71145

Keywords: melatonin, DLMO, circadian rhythm, autonomic nervous system, heart rate variability, blood pressure, phase angle, parasympathetic modulation, sleep, cardiovascular physiology, constant routine, cardiovascular