Evidence for a biological dawn and dusk in the human circadian timing system

Abstract

Because individuals differ in the phase angle at which their circadian rhythms are entrained to external time cues, averaging group data relative to clock time sometimes obscures abrupt changes that are characteristic of waveforms of the rhythms in individuals. Such changes may have important implications for the temporal organization of human circadian physiology. To control for variance in phase angle of entrainment, we used dual internal reference points ‐ onset and offset of the nocturnal period of melatonin secretion ‐ to calculate average profiles of circadian rhythm data from five previously published studies. Onset and/or offset of melatonin secretion were found to coincide with switch‐like transitions between distinct diurnal and nocturnal periods of circadian rhythms in core body temperature, sleepiness, power in the theta band of the wake EEG, sleep propensity and rapid eye movement (REM) sleep propensity. Transitions between diurnal and nocturnal periods of sleep‐wake and cortisol circadian rhythms were found to lag the other transitions by 1‐3 h. When the duration of the daily light period was manipulated experimentally, melatonin‐onset‐related transitions in circadian rhythms appeared to be entrained to the light‐to‐dark transition, while melatonin‐offset‐related transitions appeared to be entrained to the dark‐to‐light transition. These results suggest a model of the human circadian timing system in which two states, one diurnal and one nocturnal, alternate with one another, and in which transitions between the states are switch‐like and are separately entrained to dawn and dusk. This description of the human circadian system is similar to the Pittendrigh‐Daan model of the rodent circadian system, and it suggests that core features of the system in other mammals are conserved in humans.

Relationship between post-awakening salivary cortisol and melatonin secretion in healthy participants

“…Further both are affected by bright light (Scheer & Buijs, 1999;Thorn et al, 2004;Claustrat et al, 2005;Wehr et al, 2001). Despite these complementary mechanisms and functions data from this study suggest no direct relationship between the hormones in the post awakening period; relationships later in the day were not explored.…”

“…Circadian rhythms exhibit distinct diurnal and nocturnal states with an abrupt switch-like transition between sleep and waking; initiating a more gradual change in function to prepare for the day ahead: biological dawn (Morris et al, 2012;Wehr et al, 2001). Biological dawn is frequently measured by examination of post-awakening cortisol secretion, an index of circadian functioning of the HPA axis.…”

Interactions between endocrine and circadian systems

“…The circulating levels of both display robust diurnal patterns (Migeon et al 1956, Ralph et al 1971. Using the CR and FD experimental protocols, the secretion rhythms of cortisol (el-Hajj Fuleihan et al 1997, Wehr et al 2001, Aeschbach et al 2003, Scheer et al 2010) and melatonin (Dijk et al 1999, Wehr et al 2001, Cain et al 2010, Gooley et al 2011 have been shown to be under direct regulation by the circadian clock. Not surprisingly, both hormones also act as major hormonal output pathways that propagate the time signal from the SCN to various other tissues.…”

Effects of Artificial Dawn on Subjective Ratings of Sleep Inertia and Dim Light Melatonin Onset

“…A possible shift in the offset of the melatonin rhythm or suppression in the early morning was not measured. It could be hypothesized that morning light induces a larger phase advance in the offset than in the onset of the melatonin rhythm (Illnerová & Sumová, 1997;Warman et al, 2003;Wehr et al, 2001). This effect, however, is thought to be transient and only present during the first days after morning light.…”