Behind the Brain

When we sleep, we go through four phases of sleep. The first 3 phases are classified as NREM (Non-Rapid Eye Movement), while the last phase is classified as REM (Rapid Eye Movement). But how do these phases of sleep, help our brain take a break?

Stages of NREM Sleep

1. Stage 1: This initial stage of NREM sleep helps our heart rate, our breathing rate, and eye movements slow down. Additionally, alpha and theta brain waves are released.
2. Stage 2: The second stage of NREM sleep is deeply associated with formation and consolidation of memory. During this stage, the thalamus gives out short bursts of activity, known as sleep spindles. Furthermore, the hippocampus also shows signs of activeness. Both these parts of the brain are associated with memory formation and consolidation.
3. Stage 3: The third stage of NREM sleep is all about repair. During this stage, thalamus activity dies down and basic life functions such as heart rate, and breathing take over. Furthermore, healing and repairing tissue happens during this phase of sleep. Since all activity is reduced to a minimum during this stage of NREM sleep, stage 3 of NREM sleep is called deep sleep.

REM Sleep

During REM sleep, the brain becomes highly active, almost resembling a state of wakefulness. In REM sleep, the limbic system, due to its association with emotions, causes dreams. These dreams are also regulated by the reward dopaminergic system (the system that regulates dopamine based rewards). To prevent our muscles from moving during these dreams, the pons and medula, part of our brain stem, activate muscle paralysis. Furthermore, to keep our dreams fantastical, our prefrontal cortex remains inactive.

Regulation of the Sleep Cycle

The sleep cycle is regulated by several things, including internal and external factors. The most important of these is our circadian rhythm, our internal clock that influences when we are sleepy or alert. This “clock” is run by the hypothalamus, but is also influenced by outside factors such as light exposure. The circadian rhythm is what causes jet lag. For example, if you travelled from Canada to India, your sleep cycle would be delayed by 12 hours, making you sleepy during the day, and alert at night.

But what causes this feeling of “sleepiness”? Throughout the day, while you are awake, the body produces a chemical called adenosine, which builds up in the brain. This chemical attaches to receptors, and makes neurons more inhibitory, AKA, less active which causes sleepiness. While you’re sleeping, this build up of adenosine is removed. So based on the amount of adenosine in your brain, you feel either sleepy or awake.