Circadian rhythms are regular changes over a 24 hour cycle in sleep and wakefulness, eating and fasting as well as other biological processes. They are governed by an internal clock, which controls hormonal changes inside the body but is also influenced by external factors like exposure to light. Disruption in such patterns, such as long-term shift working, contributes to the development of cancers. Poor sleep may lead to changes in levels of hormones that play a part in developing cancers. DNA damage and repair mechanisms that may also be disrupted, increase cancer risks.

Taking advantage of circadian rhythms may improve the effectiveness of cancer treatments. The impact of anti-cancer drugs varies depending on when in the day they are administered. Monitoring patients to carefully time drug doses may therefore increase the effectiveness of treatments.

Almost all organisms have an internal clock that controls their day-night rhythms. A series of proteins whose concentrations rise and fall in cells due to negative feedback loops regulate these clocks. Changing environmental factors like eating or exposure to light can reset these cycles, but they still naturally fall back into a day-night rhythm even when organisms are in constant conditions. While the specific proteins may vary, the basic mechanism driving circadian rhythms in all plants and animals seems to be the same. Because it is so widely found, scientists assume day-night rhythms provide an evolutionary advantage and increase the fitness of organisms. For instance, organisms can use their clock to anticipate the daily environmental changes and optimize the timing of their cellular programs accordingly.

Each of us has our own circadian rhythms of sleeping and wakefulness across 24 hours. These vary between people and change as we age. Some people are early risers while some work and party late. Studies show that teenagers’ circadian rhythms are naturally shifted to sleeping in and staying up later. Disrupting these rhythms can contribute to ill health. Yet many parts of society are inflexible in the face of individuals’ natural patterns of wakefulness and drowsiness. For example, teenagers could benefit from later school start times which may better fit with when they are naturally alert.