Sleep means a condition of body and mind which typically recurs for several hours every night, in which the nervous system is inactive, the eyes closed, the postural muscles relaxed, and consciousness practically suspended.
In other words, Sleep is the mysterious shift in consciousness that our bodies require every day. It’s vital for our health and wellbeing, and not only do we function less well when we don’t get enough quality sleep, but it can lead to long-term health problems. That’s why we need to do all that we can to ensure that we enjoy quality sleep and deal with any sleep problems. During sleep, our heart rate drops, our body temperature falls and we experience complex changes in brain activity.
Sleep is largely controlled by sleep pressure, and the circadian rhythm, or our body clock, which is a 24 hour cycle that regulates all our biological and physiological processes. It anticipates environmental changes around us so that our bodies can adapt to them. In ideal situations, the circadian rhythm will naturally rise in the early morning, promoting wakefulness and alertness, and will reach a peak in the evening. After a waking period of around 15 hours the pressure to sleep becomes greater and greater, in other words, we get tired. With the onset of darkness, the circadian rhythm drops to the lowest level and helps to maintain sleep
Sleep patterns vary widely among animals and among different individual humans.
Industrialization and artificial light have substantially altered human sleep habits in the last 100 years.