Circadian rhythm
A circadian rhythm is a biological cycle that repeats about every 24 hours, such as the sleep-wake cycle. It is driven by an internal clock in the brain, which bright daylight keeps in step with the outside day.
What it means in practice
Most physiological functions and behaviors show an approximately 24-hour rhythm in everyday life [1]. These rhythms persist under constant conditions, with no day or night cues, but their period is then slightly longer than 24 hours, which is why researchers describe them as driven by an internal, self-sustained oscillator, a clock that runs on its own [1].
In mammals, including humans, the central pacemaker sits in the suprachiasmatic nucleus, a region of the hypothalamus in the brain [1]. Bright sunlight is its primary time cue, or zeitgeber, and nonphotic cues (cues other than light) such as physical exercise can also shift circadian rhythms [1]. Human rhythms have unusual features: under constant conditions the sleep-wake cycle and the core body temperature rhythm can drift apart, and the review's author says human studies must take this into account [1].
Jet lag shows what happens when the clock and the day disagree. A Cochrane review describes it as the body's internal rhythms being out of step with the day-night cycle at the destination [2].
Why it matters when reading nootropics research
Studies that time doses or measurements to the body clock need a way to locate it. Two hormones serve as markers of circadian phase: melatonin, which rises in the evening, and cortisol, which peaks shortly after awakening [3]. A 2025 methods review focuses on two such measures, dim light melatonin onset (DLMO) and the cortisol awakening response, the rise in cortisol shortly after waking [3]. The same review lists ambient light, body posture and exact sampling times as confounders that can affect these measurements [3].
On NootropicsWork, the term appears mostly on the melatonin page, where sleep and jet-lag outcomes are graded separately. A trial's timing of doses and measurements relative to the internal clock is part of its design, so it is worth checking when results are compared.
References
- Yamanaka Y. Basic concepts and unique features of human circadian rhythms: implications for human health. Nutr Rev. 2020.
- Herxheimer A, Petrie KJ. Melatonin for the prevention and treatment of jet lag. Cochrane Database Syst Rev. 2002.
- Skubic C, Zevnik U, Nahtigal K, et al. Circadian Biomarkers in Humans: Methodological Insights into the Detection of Melatonin and Cortisol. Biomolecules. 2025.
- National Library of Medicine (NIH). MeSH descriptor: Circadian Rhythm (D002940), scope note. not shown (MeSH 2026 vocabulary).