About an hour after falling asleep, a sleeper reaches the deepest stage of sleep, when slow delta waves dominate and waking is hardest. That sleep stage is

Late in the night, a sleeper is in the stage when vivid dreams are common and most muscles are inhibited. The stage is

In the first sleep cycle, a sleeper rises out of deep sleep into a lighter stage marked by sleep spindles, just before the first REM period. The sleep stage is

Just after falling asleep, a sleeper passes through a brief, light transitional stage and is easily woken. The stage is

Comparing the first and last REM episodes on the hypnogram, the night's pattern shows that REM periods

Across the night shown, the NREM-3 segments are mostly

The first descent from waking on the hypnogram follows this order:

In the later cycles on the hypnogram, most deep-sleep time is replaced by

Sleep spindles and K-complexes on an EEG are defining signs of

Large, slow delta waves are most characteristic of

Hypnagogic sensations, such as feeling like you are falling as you drift off, are most associated with

REM sleep is called paradoxical sleep because it combines

A child sits up screaming, seems terrified, and later remembers little; this usually happens during deep NREM sleep and is called

A teenager uses bright screens until 2 a.m. all summer, then cannot fall asleep at 10 p.m. for school. The shifted body clock is the

After flying across several time zones, a traveler feels wide awake at local bedtime and sleepy at lunch. This misalignment is

A nurse rotates from day shifts to overnight shifts, and morning sunlight on the drive home makes daytime sleep harder. The light cue affects the

In dim evening light, the pineal gland releases more of the hormone that promotes sleepiness. That hormone is

In psychology, consciousness is best defined as

A tennis player returns a fast serve before she can verbally explain how she tracked the ball. This illustrates

During anesthesia, a person reports no awareness, but brainstem systems still regulate breathing. The breathing control is best described as

The claim that sleep evolved because staying inactive at night kept ancestors safer from danger is the

The idea that sleep helps repair the body and replenish brain resources is the

A student studies vocabulary, sleeps a full night, and recalls more words the next day. This supports the sleep theory of

On a psychomotor vigilance task, sleep-deprived students most reliably show

After pulling an all-nighter, a student has more trouble holding instructions in mind while solving problems. The measured function hurt by sleep loss is