Analysis of Heart Rate During Sleep Phases and the Role of Circadian Hormones

This study investigates the relationship between heart rate variation during sleep and circadian physiological processes in an adolescent subject. Sleep is regulated by complex interactions between autonomic nervous system activity and hormonal rhythms, particularly melatonin and cortisol secretion. The objective of this project was to examine whether observable heart rate patterns recorded using a wearable device correspond to known sleep-stage behavior and circadian trends. Heart rate data was collected over a period of fourteen nights using a consumer-grade Huawei smartwatch. Recorded parameters included sleeping heart rate, sleep duration, and wake times. Observed heart rate values ranged from approximately 43 BPM during periods of stable sleep to temporary increases approaching 97 BPM. Repeating fluctuations in heart rate were identified throughout the night, suggesting cyclic physiological changes consistent with established sleep architecture. To interpret these observations, a simplified conceptual model was proposed relating sleep depth to circadian hormone balance and heart rate variation. Melatonin and cortisol trends were approximated using sinusoidal functions to represent opposing circadian rhythms, allowing development of a theoretical mathematical relationship linking hormonal influence with cardiovascular activity during sleep. Adenosine was subsequently incorporated to address an observed inconsistency between melatonin levels and the heart rate minimum at 0600hrs. While limitations exist due to indirect hormone estimation and reliance on consumer wearable technology, the results demonstrate that recognizable sleep-related physiological patterns can be inferred from long-term heart rate observations. The study highlights the potential of accessible wearable devices for exploratory analysis of sleep physiology and circadian regulation.