Sleep: we all do it, yet many of us don't have enough of it and take its effect on our health for granted.
As sleep expert Professor Colin Espie finely puts it: "Sleep is not an optional extra in life, it is a fundamental requirement."
There are numerous pieces of research linking sleep with a handful of vital processes related to health and wellbeing.
According to the US-based Sleep Foundation, sleep is essential to help maintain mood, memory and cognitive performance. It also plays a role in the normal function of the endocrine and immune systems.
But while a lot of vital processes take place during the heaviest stages of sleep, there are benefits to light sleep, too. This means that when you take that quick 20 minute cat nap in the afternoon, you're still doing wonders for your health.
There were previously five stages of sleep: one, two, three, four and REM (rapid eye movement), however in recent times stages three and four have been combined.
When a person sleeps, these stages progress cyclically from one through to REM, before starting again - but not necessarily in sequence.
A complete sleep cycle takes an average of 90 to 110 minutes. Without further ado, here's what the various stages of sleep can do for you.
Stage One
This is the brief stage between being awake and falling asleep, which lasts for roughly 5-10 minutes.
During this stage, the eyes move slowly and muscle activity slows. Many people experience sudden muscle contractions, which are followed by a sensation of falling.
Stage Two
This is the onset of sleep where eye movement stops and brain waves become slower, with only an occasional burst of rapid brain waves.
During this stage of sleep, a person's breathing and heart rate remains regular and their body temperature begins to drop.
During stages one and two, which are characterised as light sleep, some cell repair occurs - although not to the extent that it does during deep sleep.
Additionally, memory is "backed-up". Neuroscientists believe the brain transfers memories from short-term storage to long-term storage during this stage, which is essential for learning.
Stage Three / Four
At this point, a person has entered a state of deep sleep.
Blood pressure has dropped, breathing has slowed, muscles are relaxed and blood supply to muscles has increased.
Tissue growth and repair occurs, according to the Sleep Foundation.
During deep sleep, hormones are also released such as growth hormone, which is essential for body growth and muscle development.
REM
This stage occurs roughly 90 minutes after a person falls asleep. The brain is very active during this time, which is why dreams occur.
During this stage, energy is provided to the brain and body, according to the Sleep Foundation.
The body regulates levels of the hormones ghrelin and leptin, which help to not only keep our immune system in check, but also play a huge role in our feelings of hunger and fullness.
This is why sleep-deprived people might feel the need to eat more and, as a result, gain weight.
Over the course of the night, levels of the hormone cortisol, which would have dipped at bedtime, will begin to increase to promote alertness in morning.
