The Science Behind Your Internal Clock


Our circadian rhythms are important for good sleep, and good sleep is important for good health. Long-term sleep deprivation can lead to obesity, diabetes, cancer, and more Cognitive declineAmong other issues (Knutson & Van Gatter, 2008; Mogavero et al., 2021; Wennberg et al., 2017).

Under optimal conditions, the hormone cortisol peaks shortly after you wake up, setting a timer for the alertness-and-fatigue cycle that sends you back to sleep several hours later (Digmes, 2009).

But that process can easily go awry. Sunlight is the primary trigger for producing that spike in cortisol, although diet and physical activity also contribute. Without exposure to sunlight within the first hour after waking up, our circadian clock may not be properly entrained. The artificial lights we see most in the morning are usually not bright enough to get the job done (Potter et al., 2016).

And it gets worse. We see very little light in the morning and more in the evening. The artificial lights of our homes, mobile phones, and computer screens shine brightly in the evening after the sun goes down, signaling our circadian clock to keep us unnaturally awake so we don’t fall asleep early or deeply (Potter et al., 2016).

Optimal physical activity

Our sleep, circadian rhythms affect various aspects of metabolism such as hormone release, appetite, digestion and body temperature, all of which are essential for the proper functioning of our bodies (Potter et al., 2016).

Disruptions of these systems through disrupted circadian rhythms increase the risk of physical health problems including cardiovascular disease, obesity, diabetes and cancer. This occurs due to the direct effects of circadian disruption on metabolism and its influence on food preferences. People crave fatty and sugary foods more when sleep deprived (Simon et al., 2015).


As expected, the effects of circadian rhythms on our body’s functioning have implications for both physical and mental performance.

In one study, researchers found that swimmers were faster 90 minutes after the body temperature drop that occurs each evening (Kline et al., 2019). Among others, athletes have shown day-to-day (day-to-day) variability in key indicators of performance such as VO2 max, strength, flexibility, sprint speed, and reaction time (Knaier et al., 2019; Vitale & Weydahl, 2017).

Regarding mental performance, there is evidence that cognitive functions are influenced by states of wakefulness, which in turn are influenced by circadian rhythms. This is particularly true for cognitive functions that involve top-down cognitive control, such as inhibition, working memory, task switching, and psychomotor awareness (Xu et al., 2021).

mental health

Disturbed circadian cycles are the rule rather than the exception in all forms of mental illness (Baglioni et al., 2016). In one study of 121 adults receiving treatment at a community mental health center, more than 85% met criteria for two or more sleep-circadian problems (Sarfan et al., 2021).

Circadian rhythm and the relationship between mental health It seems to run in both directions. Disruptions to circadian rhythms contribute to psychiatric problems, while psychiatric problems contribute to disrupted circadian rhythms (Mayer et al., 2024).

There are several mechanisms that contribute to this relationship, including lack of daylight, sleep disruption, and its important role in emotional processing and Neuroticismand effects on the microbiome (Meyer et al., 2024).

Insomnia and hyperinsomnia (representing circadian disruption) are so common in depression that they are part of the diagnostic criteria for the condition (World Health Organization, 2019), and one-third of people with major depression experience both (Jeffroy et al., 2018).

However, with these bleak realities, we can set the stage for treating depression with interventions that target the circadian clock. There is evidence that exposure to very bright artificial light in the morning is effective in treating both seasonal depression (or seasonal affective disorder) and major depression (Lewy et al., 2006; Al-Karawi & Jubair, 2016).

As with other mental health problems, there is a strong correlation between anxiety and sleep disorders, with 24% of people with anxiety disorders experiencing insomnia and 28% experiencing severe insomnia (Ford & Camero, 1989).

Again, as with other mental health problems, causation can run in both directions. It’s hard to fall asleep and stay asleep when you’re anxious. Whereas disturbed sleep refers to impaired emotional processing during sleep, such as daytime anxiety, hyperarousal, rumination, and emotional reactivity (Van Soren, 2021).

Circadian rhythm disorder

Circadian rhythm disorders are classified International Statistical Classification of Diseases and Related Health Problems (World Health Organization, 2019) Sleep-wake disorders, as mentioned above, are often associated with different types of mental health problems rather than mental and behavioral disorders.

A circadian rhythm disorder is a problem that occurs when your circadian rhythm is out of step with your environment. They include (National Heart, Lung, and Blood Institute, 2022):

  • Advanced sleep-wake phase disorder: early going to sleep and early waking
  • Delayed sleep-wake state disorder: Going to bed too late and waking up too late
  • Disorganized sleep-wake state disorder: When sleep is broken up into short periods throughout the night and day
  • 24-hour sleep-wake cycle disorder: when the sleep-wake cycle is not 24 hours long
  • Jet lag disorder: When you cross time zones while traveling, you may not be in sync with when you arrive at your destination.
  • Shift work disorder: When you switch between day and night shifts, you may not be in sync with your work schedule or the world around you.

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