A body is a neat thing.

Eye Adaptation:

In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light. The human eye can function from very dark to very bright levels of light — its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly one billion apart. However, in any given moment of time, the eye can only sense a contrast ratio of one thousand. What enables the wider reach is that the eye adapts its definition of what is black. The light level that is interpreted as "black" can be shifted across six orders of magnitude—a factor of one million.

The eye takes approximately 20-30 minutes to fully adapt from bright sunlight to complete darkness and become ten thousand to one million times more sensitive than at full daylight. In this process, the eye's perception of color changes as well. However, it takes approximately five minutes for the eye to adapt to bright sunlight from darkness. This is due to cones obtaining more sensitivity when first entering the dark for the first five minutes but the rods take over after five or more minutes.

Stages of Sleep (NREM):

Stages of sleep In mammals and birds, sleep is divided into two broad types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The American Academy of Sleep Medicine (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called delta, or slow-wave, sleep (SWS).

NREM

Stage N1 refers to the transition of the brain from alpha waves having a frequency of 8 to 13 Hz (common in the awake state) to theta waves having a frequency of 4 to 7 Hz. This stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and hypnic jerks, also known as positive myoclonus, may be associated with the onset of sleep during N1. Some people may also experience hypnagogic hallucinations during this stage, which can be troublesome to them. During N1, the subject loses some muscle tone and most conscious awareness of the external environment.

Stage N2 is characterized by sleep spindles ranging from 11 to 16 Hz (most commonly 12–14 Hz) and K-complexes. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45% to 55% of total sleep in adults.If you sleep 7-8 hours a night, all but maybe an hour and a half is spent in dreamless NREM sleep.

Thanks wikipedia.