Behavioral manifestations such as the sleep-wake cycles are also controlled by the clock - and it is important to note that, unlike normal clocks, our biological clocks are not synchronized and do not keep the same pace.

Late at night and early in the morning, while we sleep, the epidermal (or skin) stem cell creates new cells, replacing the aged ones. This happens when the body is supposed to be at rest and not disturbed - hence the importance of keeping a sleep-wake schedule.

The skin barrier is the first line of defence for the body against microorganisms and environmental factors. Throughout the day, the skin cells operate different activities to protect and defend the body - hence why the skin is more sensitive at certain times.

Skin hydration is necessary to maintain a protective barrier against infection and dehydration. The water content of the skin has a daily rhythm, with minimum loss (through the pores of the skin) in the morning and an increased water loss during the night.

The skin has glands that produce sebum, which waterproofs and lubricates the skin and hair. Sebum excretion peaks around midday, with the lowest levels in the late evening and the morning.

Skin cells also have their own clocks, which can function autonomously. Many external environments may also reset these clocks (pollutants, temperature, humidity, ultraviolet radiation…).