![]() Delta waves appear and soon account for nearly half of the waves in the EEG trace. Stage 3 non-REM sleep marks the passage from moderately to truly deep sleep. But because people go through Stage 2 sleep several times during the cycles in a night, this is the stage in which adults spend the greatest proportion of their sleep–nearly 50% of the total time that they sleep each night. It is still possible to awaken them, even if they then report that they were really sleeping during the 10 to 20 minutes that this stage lasts during the earliest of the night’s sleep cycles. People in Stage 2 sleep are unlikely to react to a light or a noise, unless it is extremely bright or loud. The K-complex seems to be associated with brief awakenings, often in response to external stimuli. They are generated by interactions between thalamic and cortical neurons.ĭuring Stage 2 sleep, the EEG trace may also show a fast, high-amplitude wave form called a K-complex. Sleep spindles generally last 1 to 2 seconds. These bursts of activity have a frequency of 8 to 14 Hz and an amplitude of 50 to 150 µV. The theta waves characteristic of Stage 2 sleep are interrupted by occasional series of high-frequency waves known as sleep spindles. The alpha rhythm is sometimes not clearly dis- tinguishable, in fact about 10 to 20 of healthy people have little or no alpha. Stage 2 non-REM sleep is a stage of light sleep in which the frequency of the EEG trace decreases further while its amplitude increases. Each period of Stage 1 sleep generally lasts 3 to 12 minutes, They also often report having had stray thoughts and short dreams. In experiments where people are awakened from Stage 1 sleep and asked about their state of consciousness, they usually report that they had just fallen asleep or had been in the process of doing so. Though your reactions to stimuli from the outside world diminish, Stage 1 is still the phase of sleep from which it is easiest to wake someone up. Soon, the even slower theta waves begin to emerge. Figure 1.4 A typical hypnogram of a young adult showing discrete cycles of non-REM and REM sleep through the night. Then, as you continue falling asleep, the rapid beta waves of wakefulness are replaced by the slower alpha waves of someone who is relaxed with their eyes closed. After a few sudden, sharp muscle contractions in the legs, the muscles relax. Stage 1 non-REM sleep begins when you first lie down and close your eyes. Obviously, falling into a deeper and deeper sleep as the night progresses is actually a gradual, continuous process, but these four stages still provide a convenient means of describing the relative depth of NREM sleep. \), and others discussed below, are important criteria that have been used to define four distinct stages of non-REM (NREM) sleep.
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