A sudden feeling of hunger and wanting to eat may be called a hunger pang. Is there a word for a sudden feeling of tiredness and wanting to sleep?
I think you may be referring to what is generally called:
A sudden onset of drowsiness: (from www.healthgrades.com)
- Drowsiness refers to feeling sleepy or tired, or being unable to keep your eyes open. Drowsiness, also called excess sleepiness, can be accompanied by lethargy, weakness, and lack of mental agility. While most people feel drowsy at some point or another, persistent sleepiness or fatigue, especially at inappropriate times, can indicate a sleep disorder or other medical problem.
A common way to indicate a sudden feeling of tiredness is using the phrasal verb set in after tiredness (or any related term like fatigue, lethargy etc.). Also, kick in can be used.
set in A desperate tiredness set in after hours of anxious waiting.
Periods of "driving unconsciousness" usually last only a few seconds while the brain goes on autopilot. Psychologists say that when boredom and fatigue set in, a driver slips in and out of unconscious states. "Brain fade" is the familiar term.
[Popular Mechanics - Vol. 161, No. 8 - Magazine]
Also, somnolence, as a single word, suggests a state of near-sleep or a strong desire for sleep.