When the sun goes down, our body produces melatonin, a hormone responsible for drowsiness. Increased melatonin production lets us know it is time to sleep. However, sunrise produces another hormone called cortisol which increases energy levels instead. This shows how external stimuli influence sleepiness.
The body must go through the four stages of sleep approximately four to six times–what we call a good night's sleep.
Stage 1 NREM (non-rapid eye movement) sleep
It is the shortest and is referred to as a transition from "quiet wakefulness" to "light sleep", lasting about 5 minutes. On the electroencephalography (EEG), brain waves go from 8-13 Hz (alpha waves) to 4-7 Hz (theta waves) in frequency. Alpha waves are one of the waves found during wakefulness along with beta waves. The theta wave pattern increases as we enter "light sleep". It is easier to wake someone up at this stage, but if undisturbed they can quickly progress to stage 2.
Stage 2 NREM sleep
Stage 2 generally takes about 10 to 25 minutes. Your body temperature drops and your muscles, heart rate and breathing relax. Your brain waves become incoherent, which means theta waves stumble over short bursts of activity called spindles. Research also found that K-complexes primarily appear in this stage and mediate cortical arousal, our brain’s response to stimuli like sound. They can be seen as a bridge to the external world during sleep.
Stage 3 NREM sleep (delta sleep or slow-wave sleep)
The brain reacts less to stimuli, which means it is harder to wake someone up at this stage. Our brain shifts from theta waves to delta waves. Delta waves are slow recorded brain waves with a frequency of 0.1-3.5 Hz during this deep, restorative sleep. It is a very healing stage for the body and the mind and lasts approximately 20 to 40 minutes.
REM (rapid eye movement) sleep
Brain activity, heart rate, and breathing increase and become irregular. Brain waves during REM sleep are very similar to waves during wakefulness. Your muscles enter a paralysis state, also known as atonia. Your eyes move quickly, but you still cannot see anything. The brain begins the process of memory consolidation, converting recent moments into long-term memory. This stage is particularly important for cognitive development and retention and is usually when dreams come into play. Research claims that the forebrain is responsible for dreaming while the brainstem is responsible for REM sleep. There are many theories aimed at explaining the science behind dreams and their complexity. Although, as with all things science, there is yet to explore.