Our Sleep and Wake Patterns
Today we understand our sleep and wake patterns through the two-process model of sleep regulation. The two-process model delineates two principal mechanisms for the governance of sleep and wakefulness: “Process S” and “Process C.”
The homeostatic drive to sleep (Process S) is proportional to the duration of wakefulness. In contrast, Process C creates a drive for wakefulness that variably opposes Process S and is dependent upon circadian rhythms intrinsic to the individual.
Master coordination of this sleep-wake rhythm is provided by the neurons of the suprachiasmatic nuclei located within the hypothalamus.
As this intrinsic period is typically slightly longer than 24 hours in humans synchronisation to the 24-hour day is accomplished by various environmental inputs, the most important of which is light and dark exposure.
Failure to synchronise can alter the phase relationships between internal rhythms and the light/dark cycle, which may manifest in the form of circadian rhythm sleep-wake disorders and sleep loss or disruption.
Chronobiology
Chronobiology is a field of biology that examines periodic phenomena in living organisms and their adaptation to solar and lunar related rhythms. These cycles are known as biological rhythms. The one we are particularly interested in is the human circadian rhythm. When a circadian rhythm is regulated by natural environmental conditions, it is called an entrained rhythm. In humans, the main circadian clock is located within the suprachiasmatic nuclei (SCN) of the hypothalamus.
The master clock in the human brain coordinates all the cellular clocks. The location of the SCN just above the intersection of the left and right optic nerves gives the SCN the opportunity to receive light signals from the retina which it utilises in circadian rhythm entrainment.
The signals directly from the retina are used to adjust the biological clock and synchronise it within a 24hr cycle. There exists special photosensitive cells on the retina, which contain the light sensitive pigment melanopsin. The melanopsin cells are stimulated by natural day light, especially medium - long wavelength blue light. Exposure to natural daylight stimulates nerve pathways in SCN which plays a major role in synchronising the central circadian clock with the day.
Light Is Our Zeitgeber
It is widely accepted that light is the primary synchronising stimulus for the human circadian system.
The responses of the human circadian system to nocturnal light was measured as a function of the light’s intensity. The resultant dose-response curves demonstrated that most of the circadian phase resetting and melatonin suppression responses are achieved at light levels of ∼100 lux.
Before the development of the light bulb, people would have spent nearly all of their time exposed to light intensities at the minimal and maximal ends of the dose-response curve: natural daylight (>300 lux) or dim sources of light after sunset (<30 lux), including moonlight and small fires.
Today’s light environment differs considerably in the amount of time spent at intermediate light intensities (30-300 lux). Exposure to electrical light after sunset in this range is extremely common in industrialised countries, and the increasing use of light-emitting devices is a further source of evening light that is potentially disruptive to the circadian system.
Evidence Base
- Strategically timed light therapy can suppress melatonin synthesis and phase shift circadian timing in humans, indicating the suitability of timed light exposure as a treatment for circadian rhythm sleep disorders.
- Light exposure in the evening and early night prior to the circadian phase of the core body temperature minimum induces phase delays of circadian rhythms, as opposed to light during the late night/early morning which results in phase advances.
- Just as light exposure can shift circadian timing, so too can the strategic avoidance or reduction of light.
- The human circadian system is most sensitive to particular wavelengths at varying intensities.