During the night, a normal sleeper moves between different sleep stages in a fairly predictable pattern, alternating between REM and non-REM sleep. When these stages are charted on a diagram, called a hypnogram (see Figure 2), the different levels resemble a drawing of a city skyline. Sleep experts call this pattern sleep architecture.
In a young adult, normal sleep architecture usually consists of four or five alternating non-REM and REM periods. Most deep sleep occurs in the first half of the night; as the night progresses, periods of REM sleep get longer and alternate with Stage 2 sleep. Later in life, the sleep skyline will change, with less deep sleep, more Stage 1 sleep, and more awakenings.