The stages of sleep

When evaluating sleep, a distinction is made between different sleep stages. These sleep stages are based on electrical brain activity as measured by electroencephalography and the sleep scoring rules provided by the American Association of Sleep Medicine (AASM). Note that an EEG is not a registration of activity of individual neurons, but a group of neurons.

Picture of EEG cap.
These nets can range from just a few, to up to 256 electrodes, increasing spatial resolution.

Four sleep stages are distinguished: N1, N2, N3 and REM-sleep. Note that this is different from the older Rechtshaffen & Kales scoring guidelines (S1, S2, S3, S4 and REM-sleep) that are still used a lot in the presentation of sleep stages, e.g. as presented in hypnograms (a hypnogram is a graphical representation of the stages of sleep as a function of time). Since sleep stage 3 and sleep stage 4 however don’t reveal any physiological differences, this was reduced to one stage (N3) in the new sleep scoring rules.

Now what do these sleep stages represent? These sleep stages as mentioned earlier, on the one hand represent different levels of brain activity, as well as a different level of consciousness or behavioural responsiveness. In the picture below you can see how this brain activity (brain waves) looks different across different behavioral states (from relaxed wakefulness to slow wave sleep (N3 – S3/S4) and REM sleep). As you can see, the EEG signal changes across the different stages, with high frequency (‘x-axis’; you can see the lines very close to one another) and low amplitude ( ‘y-axis’, height of the lines) brain waves during the first stages of sleep and lower frequency, high amplitude waves during the deeper stages of sleep.

Based on the frequency (expressed in Hz, i.e. waves per second) different waves are distinguished, ranging from Gamma waves (very high frequency waves) to Delta waves (low frequency waves) (cfr. wave patterns).

Picture relating the different EEG signals (brain waves) to different sleep stages.

As I mentioned this EEG signal presents the activity of groups of neurons. When an EEG signal show high frequency, low amplitude activity, this means a lot of neurons are firing at different times (asynchronous activity) as opposed to when neurons are firing at the same time (synchronous activity).

Some hallmarks of the different sleep stages:

  • Stage 1 (N1): theta waves / alpha waves – eyes closed (which can be seen by electrodes places close by the eyes – EOG)
  • Stage 2 (N2): marked by spindles (short bursts of high frequency activity; a lot of research focuses on these types of brain waves, that are hypothesized to reflect cognition and/or might be involved in sleep-protective mechanisms)
  • Stage 3 (N3): the so-called slow-wave activity or deeper stages of sleep, markes by delta waves
  • REM sleep: similar EEG signal as wakefulness, but the EMG (electrodes for measuring muscle activity) reflects absence of muscle activity and the EOG reflects rapid eye movements (hence the name “rapid eye movement sleep” as opposed to the NREM sleep stages when referring to N1-N3)

I’m off to my own sleep stages now! Goodnight!