The Sleep Cycle, NREM & REM Explained Simply
So, a complete sleep cycle is (N1 > N2 > N3 > N2 > N1 > REM).
Another way of looking at this is N1 is the transition from wakefulness to sleep. N2 is light sleep. N3 is deep sleep. REM is when we are dreaming.
NREM - Non-Rapid Eye Movement Sleep
NREM can be broken down into 3 (N1, N2, N3) or 4 (N4) stages. However modern science tells us there is little in the way of significant difference between stages N3 and N4 so we can consider them both N3. Hayes N tells us these phases of sleep have different levels of brain activity according to EEG data.
· N1 (5% of a sleep cycle) has swift and irregular brain activity at very low voltage. Additionally, some muscle twitches or limb jerks might happen. This stage is when we are changing from wakefulness to sleep.
· N2 (45% of a sleep cycle) shows similar activity, but with a larger range of voltages. Conscious awareness of the environment around you disappears in this deeper sleep phase. This stage is light sleep.
· N3 (25% of a sleep cycle) is called slow-wave or deep sleep. N3 has less brain activity, but larger alterations in voltage occur than during other NREM phases. It is thought to be the most restful phase and yet this is when sleep walking and bed wetting may take place. During NREM there is little or no eye movement and any dreams people have in this phase are less vivid than during REM sleep which follows it. It is also said this stage is when tissue restoration is at its height.
REM - Rapid Eye Movement Sleep
REM sleep is typically associated with dreaming and of course rapid eye movements. When woken up during this phase people will often report remembering vivid dreams. During REM our muscles are normally paralysed (apart from in some sleep disorders such as restless leg syndrome). This is to make sure we are safe from harm should we “act out our dreams” while asleep. Hayes N says REM is also known as paradoxical sleep because the level of EEG brain activity is low, suggesting light sleep. However, it can be harder to wake someone up from REM than NREM, suggesting a deeper level of sleep.
Hayes, Dr Nicky. Understand Psychology: Teach Yourself: How Your Mind Works and Why You Do the Things You Do. John Murray Press. Kindle Edition.
Waller D (2018) Working with Insomnia: WORKBOOK.