In the fairy tale the prince’s kiss might wake up the sleeper but in real life you need some brain chemicals and neural circuitry to help. In the two-process model both the homeostatic and circadian cycles are at a point to indicate awakening. The build-up of sleep-inducing chemicals such as adenosine has declines, and the brain is refreshed with new supplies of waking chemicals. Physiological processes that follow the circadian clock are primed for awakening.
In the hour or so before awakening, the body starts to “rev up” and prepare for more activity. The body temperature starts to rise, having reached its lower level near the end of the sleep period. The blood pressure increases with increases in serum levels of ACTH and cortisol. (This is why heart attacks are more common in the morning.)
Just as sleep onset and maintenance are influenced by a complex soup of neurotransmitters, so waking up involves changes in brain chemical levels and the activity of certain neurons. Norepinephrine, acetylcholine, serotonin, and histamine are among the substances that make major level changes during the wake-up process, and orexins are critical for keeping the brain awake during the day.
From a physiological/EEG point of view, awakening involves a large increase in electrical activity in the cortex.
In the absence of an alarm or artificial awakening, the end of sleep often happens after a REM period, which is sometimes results in memorable dreams. Other times awakening happens during light sleep. When a person is in a normal routine with regular wake-up times, the awakening does not normally happen from deep sleep. If a person is forced out of deep sleep, sleep inertia can be bad.
Physically getting out of bed further enforces the “awakeness”. Different people rev up at various rates. Some need a shower and a cup of coffee to get going; other leap out of bed. Once awake, the brain is bombarded with stimuli from the environment and from within the body. A homeostatic mechanism kicks in and the waking brain keeps itself awake. Scientists have found sleep-inhibition mechanism in the brain: there are separate mechanisms for inhibiting REM and NREM sleep.