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!
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.
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%.
Now we know that we all sleep in cycles and ideally we should be getting 5 of these cycles consistently every night. To calculate your cycle length write down the amount of sleep you have had over a few nights and divide by 5. By careful not to include any nights where you may have had only 4 cycles!
Now that you know your cycle length write down on a piece of paper your ideal wake up time. Work back 5 cycles from that wake up time and add 15 minutes to get to sleep. That should be the time you go to bed.
Here is an example:
Night 1: 490 minutes divided by 5 equals 98 minutes
Night 2: 495 minutes divided by 5 equals 99 minutes
Night 3: 392 minutes divided by 5 equals 78 minutes
Night 4: 495 minutes divided by 5 equals 99 minutes
Night 3 looks like a 4 cycle night so exclude that night. I think a good estimate for this person is 5 cycles of 99 minutes.
If their wake up time is 7am then work back from that time 5 cycles of 99 minutes which would be 10.45pm. Add another 15 minutes to get to sleep and their go to bed time would be 10.30pm.
The amount of sleep that you need varies from person to person. This is the same for the actual time that you naturally start to feel sleepy. However the fundamentals of sleep are consistent and by understanding these you should able to calculate the time you should ideally be going to bed.
We hope this helps a little towards a better night sleep!