Module 7: Human Circadian Rhythms and Entrainment

This set explores the mechanics of human circadian entrainment, detailing the history of light studies, the diagnostic markers of circadian blindness, and the impact of photic and non-photic stimuli.

Aschoff & Wever (1960-70s)

Researchers who conducted early 'bunker' studies in temporal isolation which led to the initial, incorrect conclusion that humans are entrained by social cues rather than light.

Czeisler

The researcher who demonstrated that when light is strictly controlled, human subjects entrain to the light-dark cycle, disproveing earlier theories about the dominance of social cues.

Circadian blind

A condition where there is no photic input to the circadian clock due to enucleation or total retinal degeneration; it is diagnosed if bright light fails to suppress plasma melatonin levels.

Visually blind

Individuals with no conscious light perception who may or may not still possess photic input to the circadian clock.

Melatonin suppression test

A test for circadian blindness where bright light exposure is used to determine if the retina-to-pacemaker-to-pineal pathway is intact; a lack of suppression indicates 'circadian' blindness.

Logistic dose-response curve

The curve describing light efficacy, showing that half of maximal phase delay and melatonin suppression from evening bright light ($9000\,\text{lux}$) can be achieved with just over $1\%$ of that intensity ($100\,\text{lx}$).

DLMO (Dim Light Melatonin Onset)

The time of day when melatonin secretion begins under dim light conditions, used as a circadian phase marker.

Tau ($\tau$) in totally blind individuals

The free-running period of the internal clock in those without photic entrainment, typically found to be approximately $24.6\,\text{h}$.

Lombrives Cave study

A human group isolation experiment in France where 15 adults lived for 40 days without external time cues, resulting in varied free-running cycles ranging from $19.5$ to $86.4\,\text{h}$.

Tasimelteon

A melatonin receptor agonist tested in randomized, double-masked trials that was shown to effectively entrain non-entrained blind individuals to a $24\,\text{h}$ day.

PRC (Phase Response Curve) to light

A curve showing the relationship between the timing of light exposure and the resulting phase shift, using melatonin midpoints as the phase marker.

Actigraphy

A method used to detect rest-activity rhythms, such as the finding that 18 out of 19 blind subjects maintained a significant non-$24\text{-h}$ rest/activity rhythm.

T-cycle entrainment paradigm

An experimental protocol involving a $23.66\,\text{h}$ sleep-wake schedule used to demonstrate that daytime exercise can facilitate the advancement of the circadian clock.

Non-24h sleep-wake disorder

A disorder characterized by a circadian rhythm that fails to entrain to the $24\text{-h}$ day; it occurs in nearly all circadian blind individuals and can also occur in normally sighted people.

Nonphotic zeitgebers

External cues other than light, such as exercise, social contacts, and food availability, which have varying and often weaker effects on human circadian entrainment.