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:
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.
The word hypnagogia is a perfect one to talk about in a discussion of sleep onset, as it refers precisely to the transitional state between wakefulness and sleep.
All sorts of really interesting things can happen during this brief state, including what's known as hypnagogic hallucinations.
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:
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).
To test just how dramatic the change in consciousness is at the moment of sleep, researchers once conducted a series of experiments involving strobe lights, a microswitch, and tape.
Adult volunteers took turns laying down on a gurney with an extremely bright (we're talking 50,000 lux) strobe light hanging six inches above their eyes. The researchers taped the participants eyelids open, and put a small switch against their finger with the instructions that every time the participants saw the strobe light flash they must press the switch. Easy enough, right? I mean, we're talking about a 50,000 lux strobe light flashing 6 inches in front of your face. How could you miss that?
Well, despite having their eyes open the entire time, the participants did miss the strobe light flashes whenever they were asleep while they went off. When brain wave recordings being taken simultaneously showed the participants to be awake, not one failed to press the switch after a flash, without exception.
But whenever the brainwave patterns showed the participant had slipped to non-REM stage 1 sleep (the lightest stage of sleep), the subjects failed to press the switch and even denied that there had been a strobe flash after the fact! This was invariably true even if the flash followed the brainwave change by less than a second, or in other words, right after sleep onset.
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!
Welcome! This site is continuously being created by students of Dr. William C. Dement's Sleep And Dreams course at Stanford University.
We made this site as a call to action for people all over the world to live healthier, happier, safer, and more productive lives by learning about their own sleep. We have faith that reading the information provided on this site will motivate you to be smart about your sleep deprivation and strategic about your alertness in order to live life to your fullest, most energetic potential.
In fact, we challenge you to do so! What do you say, are you up for the challenge?
Dr. Dement's pioneering textbook has been the core text for Sleep and Dreams since 1980, but it has just recently been made available to the wider public for the first time.
In it you'll find a more detailed account of the most important things you need to know about sleep, alertness, dreams, and sleep disorders. Studies, statistics, plus plenty of Dr. Dement's classic anecdotes painting the history of sleep medicine.
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