Neurons are firing at different times. A person is sleepy, already sleeping, or in a sleep transition. It can be observed in from the hippocampus. Theta rhythm is also observed in REM sleep. Therefore, the activity is synchronized.
Waves are large and slow.
The time spent in REM sleep is represented by a light-blue bar. The amount of stage 2 slow-wave sleep increases progressively until it completely occupies the slow waves period toward the end of the night. Paulo and post doctoral fellowship by the University of California, Los Angeles.