Circadian Rhythms

What are circadian rhythms?

Circadian rhythms are the natural biological processes that regulate our sleeping and waking. In humans and most other animals, our circadian rhythms are on an approximate 24 hour cycle. Circadian Rhythms are generated by our bodies, but they do follow external cues such as light and temperature. It is sometimes called a sleep/wake cycle, and when someone says that they are a night or morning people it normally refers to when they feel most alert based on their circadian rhythm. The internal cycle is controlled by the hypothalamus in the brain, whereby when it's dark or light the eyes send a cue to the hypothalamus telling it to feel tired (and therefore release melatonin) or awake [1] Jump to reference section: [1] .

How do our circadian rhythms affect us?

Circadian rhythms work in tandem with regular cycles, so if people maintain regular sleeping and food habits, they can maintain a consistent rhythm. Our circadian rhythm can be disrupted by things like jet-lag and one-off occurrences of staying up or waking up late. Other deviations from normal routine such as midnight snacks or all-nighters can also disrupt our circadian rhythms. While we can tolerate occasional disruptions, chronic disruptions to our natural circadian rhythms can cause health problems.

Circadian rhythms affect day to day bodily functions such as sleep, the release of hormones, eating habits, and body temperature. Irregular rhythms have been linked to several different chronic conditions such as sleep disorders, obesity, depression, and seasonal affective disorder (SAD). There is significant evidence that mental health conditions correlate with disrupted circadian rhythms [2] Jump to reference section: [2] .

Are they related to genetics?

Our genetics can affect circadian rhythms in subtle as well as dramatic ways. Numerous genetics studies have shown that genetics can influence whether some people are ‘night-owls’ and others are ‘early birds’. Several genetic variants have also been linked to severe sleep disorders and risk of conditions such as schizophrenia.

The authors of the largest study on ‘chronotype’ (morning person versus night owl) found more than 300 genetic variants associated with being a ‘morning person’.

One of the strongest effect variants was rs150812083. People with the ‘CG’ variant at this allele were, on average, 1.4 times more likely to be a morning person, while people with ‘GG’ were more than twice as likely. This genetic variant is rare, so most people will carry CC.

See what variant you carry at this position:

This genetic variant likely affects the gene called PER3 - other rare genetic variants in PER3 have also been associated with more severe sleep changes including ‘Familial Advanced Sleep Disorder’ which often results in very early sleeping and waking patterns [3] Jump to reference section: [3] .

In this same study, the authors reported that people with genetic pre-disposition to being ‘morning people’ were also at lower risk of schizophrenia:

“We saw evidence that being a morning person confers a liability to lower risk of schizophrenia and greater subjective well-being” [4] Jump to reference section: [4] .The PER2 gene has also been shown to affect our circadian rhythm. A genetic variant in rs1801260 has been shown, first in a set of Japanese people, and subsequently in other studies, to affect preference of waking and sleeping time. People who carry one or more ‘G’ variant at this position are more likely to be late to bed and late to rise, on average, compared to people carrying the ‘A’ variant [5] Jump to reference section: [5] . See what variant you carry at this position:

The bottom line

Several genes are linked to how our circadian rhythms function, and these are also closely related to other conditions. There has even been some research, in mice, to suggest that circadian rhythms could be altered (for example, to ease the effects of Jet Lag) with drugs that target important circadian rhythm genes [6] Jump to reference section: [6] . While the link between genetics and circadian rhythm is clear, factors such as the amount of sunlight per day, and eating habits still play the primary role. Maintaining a regular routine can help stabilise your internal clock and your health.