We fixate on diet and exercise, but in the West, of at least equal importance is sleep. I've ruminated on the importance and nature of sleep a few times in the past. Only a day or two ago there was broad media coverage of the health implications of blue light from modern media and its impact on our circadian rhythm. Sleep and our circadian rhythm are every bit as important as the 'diet and exercise' tenets of health and worthy of greater understanding.
The diagram above was appropriated from a site which has a good breakdown of the sleep and shows that idea of a 'solid eight hours' is indicative of sleep debt.
The same site also lists the following four stages of sleep:
- Non-REM sleep
Stage N1 (Transition to sleep) – This stage lasts about five minutes. Your eyes move slowly under the eyelids, muscle activity slows down, and you are easily awakened.
Stage N2 (Light sleep) – This is the first stage of true sleep, lasting from 10 to 25 minutes. Your eye movement stops, heart rate slows, and body temperature decreases.
Stage N3 (Deep sleep) – You’re difficult to awaken, and if you are awakened, you do not adjust immediately and often feel groggy and disoriented for several minutes. In this deepest stage of sleep, your brain waves are extremely slow. Blood flow is directed away from your brain and towards your muscles, restoring physical energy.
- REM sleep
REM sleep (Dream sleep) – About 70 to 90 minutes after falling asleep, you enter REM sleep, where dreaming occurs. Your eyes move rapidly, your breathing shallows, and your heart rate and blood pressure increase. Also during this stage, your arm and leg muscles are paralyzed.
- You may think that once you go to bed, you soon fall into a deep sleep that lasts for most of the night, progressing back into light sleep in the morning when it’s time to wake up. In reality, the sleep cycle is a lot more complicated.
When you chart the sleep stages over the course of the night, the result looks like a city skyline—which is why it is called "sleep architecture".
During the night, your sleep follows a predictable pattern, moving back and forth between deep restorative sleep (deep sleep) and more alert stages and dreaming (REM sleep). Together, the stages of REM and non-REM sleep form a complete sleep cycle. Each cycle typically lasts about 90 minutes and repeats four to six times over the course of a night.
The amount of time you spend in each stage of sleep changes as the night progresses. For example, most deep sleep occurs in the first half of the night. Later in the night, your REM sleep stages become longer, alternating with light Stage 2 sleep. This is why if you are sensitive to waking up in the middle of the night, it is probably in the early morning hours, not immediately after going to bed.