Circadian Rhythm Explained

Your circadian rhythm is the roughly 24-hour internal cycle that governs when you feel sleepy and when you feel alert. It is driven by a master clock in the brain, the suprachiasmatic nucleus in the hypothalamus, which coordinates a network of smaller clocks throughout the body. This rhythm influences far more than sleep: it shapes body temperature, hormone release, digestion, alertness and mood across the day. Left to its own devices the human clock runs slightly longer than 24 hours, so it relies on external cues - most importantly light - to stay synchronised with the day-night cycle. When that synchrony holds, sleep tends to come easily at the right time and energy feels steady.

Light is the single most powerful cue, or zeitgeber, that sets your body clock. Specialised receptors in the eye detect brightness and send timing information to the master clock, telling it whether it is day or night. Morning daylight advances the clock and reinforces daytime alertness, which is why getting outside early in the day is one of the most effective ways to keep your rhythm anchored. Bright light in the evening, including from screens and overhead lighting, does the opposite: it signals daytime to the brain and can delay the clock, pushing back the natural onset of sleepiness. Managing light across the day is the foundation of a stable rhythm.

Melatonin is the hormone that communicates darkness to the body. As evening light fades, the pineal gland begins releasing melatonin, which rises through the night and signals that it is time to wind down towards sleep. Bright artificial light in the evening suppresses this release and can delay the timing of the whole rhythm. Melatonin does not force sleep so much as open the biological window for it. In the UK, melatonin is a prescription-only medicine, not a food supplement, and should only be used under medical guidance. The practical, non-medical lever most people have is light: dimming evening lighting and getting daylight in the morning supports the body's own melatonin timing.

Jet lag is the classic example of a circadian rhythm out of step with the environment. When you cross time zones, your internal clock still keeps the schedule of where you came from, so you feel sleepy and alert at the wrong local times until it gradually realigns - usually around a day per time zone crossed. Eastward travel, which requires advancing the clock, tends to be harder than westward travel. The most effective way to adjust is to use light strategically, seeking daylight at the right times for your destination, while keeping meals and sleep timing aligned with local time as soon as you arrive.

Shift work, especially night and rotating shifts, asks people to be awake and active when their body clock is pushing for sleep, and to sleep during the biological day. This chronic misalignment can make sleep shorter and lighter and is recognised as a genuine challenge for health and alertness. Helpful strategies include keeping the bedroom very dark for daytime sleep, using bright light during night shifts to support alertness, sticking to consistent routines where rotas allow, and being cautious with caffeine timing near the end of a shift. Anyone struggling significantly with shift work should discuss it with an occupational health service or GP.

The most reliable way to support your circadian rhythm is consistency. Keep regular wake and sleep times, including at weekends where possible, get bright daylight early in the day, lower lighting and screens in the evening, and time meals, exercise and caffeine to reinforce day-night patterns. Food supplements are not medicines and cannot reset a body clock; minerals such as magnesium, which contributes to normal psychological function and the reduction of tiredness and fatigue, may sit within a wider routine but are not a substitute for light and timing. If your rhythm feels persistently disrupted and affects daily life, speak to a GP, as circadian rhythm disorders can be assessed and managed.