Sleep onset latency, also known as sleep latency or SOL, is simply the time it takes for you to fall asleep. In other words, the time between full wakefulness to usually the lightest of the non-REM sleep stages. In an original set of sleep studies around sleep latency, the time it took participants to fall asleep was directly correlated to how sleep-deprived they were. The typical test used to measure sleep latency is the Multiple Sleep Latency Test (MSLT), which is designed to measure how long it takes someone to fall asleep in a quiet environment. A physician might ask someone to go through an MSLT if they report excessive daytime sleepiness since it’s a good tool for diagnosing sleep disorders like sleep apnea, narcolepsy, and others. Sleep latency is directly related to sleep efficiency. If a person spends the majority of his/her time in bed actually asleep, that’s considered sleep efficient. A sleep efficiency score of 85% is normal, while anything above 90% is considered very good. If a person is able to fall asleep quickly, chances are a good night’s sleep will follow since the two sleep qualities go hand in hand. Sleep is divided into two main phases – the light sleep, non-rapid-eye-movement stages, and the REM stage. A person who falls asleep relatively quickly is more likely to move efficiently through all the stages of the sleep cycle. Your sleep latency will fall into one of four categories.