Sleeping people are difficult to engage but easy to monitor, meaning that we know a great deal about what happens in the body and brain during our restful hours but little about the actual psychology of slumber.
One of the most interesting stages is the transition into sleep, where we can sometimes detect that our mind is changing as we slip into unconsciousness. These changes are known as the hypnagogic state and are when hallucinations are particularly common because the mind starts to ‘free up’ in poorly understood ways.
A new study has taken an interesting approach to try and understand the nature of this twilight period by using the biological measures to monitor how ‘far gone’ people are as they drift off, and then gently waking them to ask how their mind has changed.
The research team, led by Chien-Ming Yang from the National Chengchi University in Taipei, asked 20 participants to have an afternoon nap in the sleep lab while they were wired up to an EEG machine to measure electrical activity in the brain, with additional electrodes to measure eye movements, heart rate and muscle jerks.
As the participants drifted off they were awakened at different times: either just after eye-closing, the onset of ‘stage 1’ sleep where you’re still aware of the external world, the onset of ‘stage 2’ sleep where awareness starts to diminish, and after five minutes at ‘stage 2’ where awareness should have largely disappeared.
After wakening, participants were asked questions about their perception of being asleep and the experience of their own minds: “Did you fall asleep?”, “Did you see any visual images?”, “Were you able to control your perceptual experiences?”, “How real did any of the experiences seem to you?”, “How well were you able to control your thoughts?”, “Were your thoughts logical?” and several questions to try and capture the conscious experience of sleep onset.
The experience of having control over your own thoughts and how coherent and logical they seemed to begin to change almost as soon as the participants closed their eyes and they continued to seem slightly more unusual and autonomous as time went on.
However, as soon as ‘stage 2’ sleep began there was a step change into a state of mind where thoughts became markedly freewheeling, illogical and seemed to have a life of their own.
In contrast, awareness of the outside world remained largely present until ‘stage 2’ kicked in, at which point it quickly dropped off.
Most interestingly, the perception that ‘I was asleep’ when woken was most associated not with a reduced awareness of the surrounds, but instead largely relied on the experience that the sleeper no longer had control over their increasingly illogical thoughts.
In other words, we seem to know when we’ve been sleeping because we’re quickly drawn back into the world of controlled, logical thought after gently drifting in fantasy.