Sleep Onset: What Happens When We Fall Asleep?
Brain Waves, Alpha Rhythms, Perceptual Disengagement, & More

Written by Kevin Morton with adaptations from the Stanford Sleep Book

You may think you can tell when someone falls asleep just by looking at them, and sometimes you may be able to, but can you pin it down to the exact second? Sleep onset is actually a really intriguing topic to consider. What exactly happens at that critical moment when wakefulness suddenly turns into sleep?

Sleep researchers have pinned it down to a few key things. They're listed below, and you can click on each link to jump lower in the page and find out more:

Brain Wave Alpha Rhythms

When the functional state of the brain shifts from wake to sleep, the most reliable and accurate indicator of this sleep onset moment is a characteristic change in brain wave patterns.

When we're awake our brain generates a highly characteristic pattern known as an alpha rhythm. A brain wave pattern qualifies as an alpha rhythm if its oscillation is between 8 and 12 cps (cycles per second, or hertz, as is now commonly used).

Interestingly, while an alpha rhythm is present throughout wakefulness it actually is enhanced and even easier to detect when we close our eyes. This is wonderfully convenient because, one, closing ones eyes necessarily precedes sleep and, two, alpha waves are not present during sleep. So when looking at the brain waves of someone resting their eyes during polysomnography, it's that much easier for a sleep researcher to see the change from alpha rhythm to no alpha rhythm.

How easy is it? Well, some individuals have less prominent alpha rhythm that's a bit harder to make out, making the brain wave identification of sleep onset a little less precise. But within a few seconds, sleep onset can be unambiguously determined using brain waves in any case. Check out what it looks like below:

Slow Eye Movements

Another common feature of sleep onset is the occurence of slow, back and forth eye movements. These movements sometimes begin a few seconds before the EEG change in alpha rhythm if the eyes are closed leading up to sleep, and generally continue for several minutes after sleep onset.

In contrast to the rapid movements our eyes make during wakefulness and REM sleep, the slow eye movements of sleep onset aren't always binocularly syncronous (i.e. the left eye doesn't always move exactly with the right eye).

Perceptual Disengagement

Perceptual disengagement from the environment, one of the two main defining qualities of sleep, is the most salient feature of sleep onset. The occurence of sleep initiates a total lack of conscious awareness of the outside world for any stimuli that doesn't elicit arousal. So when a stimulus fails to elicit a response it would always elicit in the waking state, we know that person is asleep. Check out the experiment below to get a better sense of what I mean here.

Talk about amazing evidence of our sensory disengagement while we sleep! But how is it exactly that we can be completely unaware that a 50,000 lux strobe light flashed half a foot ahead of our open eyeballs? Further research has shown that the electrical response of our relay neurons responsible for transmitting sensory information to appropriate parts of the brain actually almost completely disappears during sleep. The brain doesn't get told because the messengers can't deliver the message!