system
Given the following post, respond as if you're commenting on Reddit:
human
Why are some people "deep sleepers" whilst others are "light sleepers?"

Is it possible to impact how `deeply` or how `lightly` you sleep? What is the role of fatigue in sleep and how large a factor does it play in your sleep patterns?
gpt
There are two different ways you could define sleep depth. One in terms of the actual stage of sleep the brain is in. The other in terms of how difficult it is to awaken someone (e.g., using a loud noise).

Sleep is usually divided into four stages. There is rapid eye movement (REM) sleep and three stages of non-REM (NREM) sleep, called N1, N2, and N3. Generally, we consider N3 to be deeper sleep than N2, which is deeper sleep than N1. This is because upon falling asleep, people tend to progress from N1 to N2 to N3, becoming progressively harder to awaken. The deeper the stage of NREM sleep, the more high-amplitude slow-wave activity you see when you measure brain electrical activity using EEG.

When individuals are sleep deprived, there is an increase in the depth of NREM sleep and an increase in the amount of slow-waves. Consequently, people who have a large background sleep debt will tend to sleep more deeply and be more difficult to awaken. 

Across a night of sleep, there is a progressive decrease in the depth of NREM sleep and the amount of slow-waves, as well as an increase in the frequency of spontaneous awakenings (people always have many brief awakenings at night, but these are usually too short to remember). This pattern is also affected by the body's circadian clock. Specifically, the clock promotes REM sleep and discourages deep NREM sleep around the time of the core body temperature minimum, which is usually an hour or two before morning awakening. In individuals who are sleeping at unusual times of day or at unusual circadian phases, the usual progression from deep NREM sleep early in the night to increased REM sleep later in the night is altered. In turn, this would affect sleep depth.

It's difficult to directly compare depth of REM sleep to NREM sleep, since they correspond to such different brain states. In terms of how difficult it is to awaken someone using a loud noise, REM sleep is similar to N2.

Of course, there are many other factors that can affect sleep depth.  Some individuals are simply less sensitive to environmental cues -- perhaps due to differences in sensory pathways. Alcohol tends to increase the depth of NREM sleep early in the night but makes sleep lighter and more fragmented in the second half of the night. Caffeine decreases sleep depth at night, with a measurable effect even if the caffeine is taken in the morning. Sleep disorders also affect sleep depth; severe sleep apnea, for example, generally prevents entry into deeper stages of sleep due to constant brief awakenings.