A normal night's sleep is made up of five cycles and each cycle lasts approximately 90-100 minutes. This equals between 7 hours 30 minutes and 8 hours 20 minutes of sleep in total.
It’s perfectly possible to survive on less than an average of 5 nightly sleep cycles in the short-term (just ask anyone with young children) however there is mounting scientific evidence that less than 6 hours sleep (or 4 or less cycles) will have health consequences.
Each sleep cycle of 90-100 minutes is split into a number of stages:
Stage One - As you begin to doze off you are still partially awake. You can be disturbed easily and may suffer from sudden jerks, such as thinking you are kicking a football, as your mind starts the transition from awake to asleep.
Stage Two - This is another stage of lighter sleep but your brain now demonstrates sudden increases in brainwave frequency called “sleep spindles”. Your brain then starts to slow down, your heart rate also slows and your temperature decreases.
Stage Three and Four - These are the deep sleep stages with stage four being deeper than stage three. Your brain has started to produce slower delta waves and you won’t experience any muscle or eye movement as your body becomes less responsive to external stimuli. A further increase in delta waves takes you to the deepest part of your sleep. It would be very difficult to wake you up in this sleep stage and if you were awoken you would feel initially disorientated. For the body and mind this is a highly restorative part of sleep.
Rapid Eye Movement Sleep - This is the final stage of sleep and it increases proportionately for every sleep cycle you go through each night. This is often the reason why you can remember more dreams from the latter portion of your sleep. The deepest, most physically restorative stages take up a larger proportion of the earlier cycles, which is why you can survive in the short term on less sleep. REM is still a very important part of sleep (as it's when the brain consolidates and processes information from the day and stores it in long-term memory), as well as performing general brain housekeeping!
The following diagram is called a hypnogram and this example shows the different stages of sleep, and the typical length of time within each stage, in each cycle:
You can clearly see the five cycles of sleep illustrated, including the fact that there is a greater proportion of deeper sleep in the first part of the night, and a greater proportion of REM sleep in the later part. You can also see that there are at least four occasions each night when you're in a very light stage of sleep. These are the times you typically wake to use the toilet or are disturbed by external noises
The percentage of sleep in each stage varies, depending on age, with children and infants spending up to 50% of their sleep in the REM stage. Whereas in adults it’s thought to be nearer 20%.
So each of us has an optimal five sleep cycles we should seek to achieve each night and those cycles should synchronise to our circadian rhythm to achieve optimal sleep. However, there are things that get in the way of this, for example our environment, our routine (or lack of routine) and our body and mind.