Well maybe not 90 minutes total but it has been found that planning your sleep in 90 minute increments will help you get a better night sleep.
You may know that when you sleep, your body goes through five distinct stages, of sleep. These stages last a total of about 90 to 100 minutes, they will then repeat continuously until you wake.
This stage is where you transition into wakefulness and sleep. Some people believe that this stage only happens once, but it will happen every time that your body enters a state of wakefulness, even if you are not aware that you are awake and including when waking in the morning. This stage lasts for about five minutes before you move onto stage two. People, who suffer from insomnia or apnea, may remain in this stage for a longer period.
The brain waves during this stage is predominantly Theta waves but some Alpha waves also occur. Your eyes may begin to roll slowly during this stage of sleep.
In Stage two, sleep peaks of your brain waves become higher, these are called sleep spindles. The spindles can be followed by k-complexes (where the peaks suddenly descend and then pick back up). Theta waves can also be found in this stage. Stage two is a period of light sleep and initially only lasts a few minutes. Gradually though, stage two sleep increases to take most the sleep cycle. During this stage your heart rate will slow and your body temperature will decrease as your body prepares to enter a deep sleep.
Stages Three and Four
These two stages make up your Delta sleep and occur when you are at the deepest sleep. These stages are longest in the first two sleep cycles and are when your body achieves the greatest amount of restoration. If you have not slept in a while, then this is the sleep that your body will crave. Delta waves will be most prevalent in these stages; however theta waves are also present. It is believed that Stage three is extremely short and will transition into stage four very quickly.
In children, delta sleep takes up about 40% of their sleep time and it will be very difficult to wake them from this stage.
Stage four will then move to stage three and then to stage two before moving onto the next stage.
During this stage your brain becomes very active. Your eyes begin to move rapidly from side to side, your heart rate and respiration speed up, and your muscles (face, fingers, and legs) may twitch. Groups of voluntary muscles may also become paralysed during this stage and due to increased brain activity, you may begin to hallucinate, or dream. The first period of this stage five can last ten minutes but this can gradually increase. Beta waves, which are in high occurrences while awake, are most prevalent here during the sleep cycle.
After Stage five the body will usually move on to stage two.
Sleep specialists have named the first four stages as Non-REM or NREM and the last stage as REM. REM stands for Rapid Eye Movement and is named due to the increase eye movement during stage five.
Your sleep cycle is pretty variable and can be affected by stress levels or certain medication which you may take. If you are not consistent with when you go to sleep and how long you sleep, your body might vary the cycles during sleep or even the length of certain stages. It is very important to stick to a schedule, such as going to sleep at midnight and always waking up at 6AM. This is about four sleep cycles and should be enough sleep for your body.
If you have sustained some type of injury or think you may be coming down with a sickness, you may require more sleep, but always make sure to try wake up during stage two sleep.
If you wake during any other stage, especially delta sleep, you will most probably feel as though you have not slept at all.