Circadian Biology Explained

How the clock works

Established. The system that keeps this internal time is called a biological clock. NIGMS describes it as made of proteins encoded by thousands of genes that switch on and off in a set order. [3] In vertebrates, including humans, the master clock sits in the brain, in a group of nerve cells called the suprachiasmatic nucleus (SCN). The SCN adjusts production of the hormone melatonin according to the light the eyes receive, and it helps keep clocks in other organs in step. [3]

The 2017 Nobel Prize in Physiology or Medicine went to Jeffrey Hall, Michael Rosbash and Michael Young. Working in fruit flies, they isolated a gene for the daily rhythm and showed its protein builds up in the cell at night and is broken down during the day. [4] The Nobel committee adds that clocks work by the same principles in other multicellular organisms, including humans, and that the clock regulates behavior, hormone levels, sleep, body temperature and metabolism. [5] The fruit-fly finding is basic science. It is not evidence that any product or schedule improves health.

What sets the clock

Light and dark have the biggest influence on circadian rhythms. Food intake, stress, physical activity, social environment and temperature also affect them. [1] Researchers call these outside cues time signals. How each cue works, and how large its effect is in daily life, differs by person and setting, and some cues are better studied than others. See Light Timing and Circadian Research and Chrononutrition Research for the human evidence on two of them.

When the clock and the outside world do not match

NIGMS lists several causes of mismatch: variants in clock genes, neurological diseases such as Alzheimer's disease, travel across time zones, shift work, and light from electronic devices at night. [6] The Nobel committee describes jet lag as a temporary mismatch between the outside environment and the internal clock, and says there are indications that chronic misalignment is associated with higher risk of various diseases. [7] Note the wording: "associated with" and "indications" describe observational patterns. They do not show that mistimed rhythms cause a particular disease in a particular person.

NIGMS also states that short-term misalignment may bring drowsiness, poor coordination and trouble with learning and focus, and that long-term sleep loss and continually shifting rhythms can raise risks of conditions such as obesity, diabetes, mood disorders and heart problems. [8] Sleep loss and shifting rhythms often occur together in real life, which makes it hard to separate their effects. For the workplace evidence, read Shift Work and Circadian Misalignment Research.

Keeping fields, tools and claims apart

• Field: chronobiology, the study of biological timing.
• Mechanism: feedback loops of clock proteins inside cells.
• Measurements: melatonin timing, body temperature, sleep records. These are signals of clock timing, not health outcomes. See Biomarkers vs Clinical Outcomes.
• Marketing claim: "reset your clock" products or plans. A mechanism shown in flies or cells does not prove a consumer product works.

Where to go next

For the person-to-person differences in preferred sleep times, see Chronotype: What Research Shows. For sleep problems linked to timing, see the plain-language guide to circadian rhythm disorders, and for everyday sleep basics see how sleep works.