Sleep restriction, also known as time in bed restriction, is a component of Cognitive Behavioral Therapy for Insomnia (CBT-I) that helps you increase how long you are able to sleep over time. The basic process has 4 steps.
Set a time limit (greater than 5.5 hours) on how long you can spend in bed, based on how long you are usually able to sleep. Once that time is up— regardless of how much you slept— get out of bed. After a week with a Sleep Efficiency of 85% or more, increase the time limit by 15-30 minutes. Repeat these steps until you reach your goal sleep time.
Sleep Efficiency is a measurement of how much time you're spending asleep relative to the time you spend trying to sleep, and is calculated as [Hours Spent Asleep]/[Total Hours in Bed]. Therefore, if someone spent 10 hours in bed, but only 6 of those hours were spent asleep then their sleep efficiency would be 6/10 = 60%.
Sleep efficiency is important for two reasons. The first is that increasing your sleep efficiency will help you associate your bed more strongly with sleep and thus allow you to sleep better. Furthermore, having a sleep efficiency for each night gives you an idea of how you slept and allows you to compare different nights even if you spent different amounts of time trying to sleep.
As you can see in the figure above, by restricting your sleep you are able to increase your sleep efficiency and spend less time lying in bed trying to sleep. Essentially getting more bang for your buck sleepwise. Once your average sleep efficiency is 85% or higher, you can start to incrementally increase the time you spend in bed.
You should start by calculating your average sleep time over the course of the week -- not how long you spend in bed, but the time you spend actually sleeping. A sleep journal can be helpful for getting the numbers you need, and can also help you keep track of your sleep restriction goals and progress. Once you have your initial figure you use the steps in the graphic below, substituting in your own average, to properly limit your time in bed.
You are probably used to the adage “people need 8 or more hours of sleep to function,” and while there is nothing bad about this fact, it is not necessarily true. Research shows that getting at 5.5 hours of sleep can give you similar levels of performance as getting more. This time period is called core sleep and contains important stages of sleep. For that reason, you should not restrict your sleep under 5.5 hours when doing your bed restriction. Core sleep is also important to keep in mind generally, because stressing about not getting 8 hours can create worries that negatively impact your ability to fall asleep.