In people who have been deprived of sleep the brain actually slows down. Advanced imaging allows scientists to measure the rate of metabolism – glucose consumption – inside the head. The rate declines the most in the thalamus, inferior temporal cortices, and the prefrontal cortices. In other words, the "higher" brain functions decline when you haven’t slept.
Ethical behavior is also more difficult (or at least less common) among the sleep deprived. When tired, we are more likely to cheat. The lower energy in the prefrontal cortex seems to make it harder to resist temptation.
What causes sleep debt? Insomnia could be a cause and we have seen daytime sleepiness is a necessary symptom of legitimate insomnia. Most sleep debt is caused by behavior and competing demands on or preferences of the sleeper. "Burning the candle at both ends" by not budgeting enough time for bed is the proximate cause of sleep deprivation in most common life.
It is worth noting that sleep debt and daytime sleepiness are not the same thing. People with sleep debt can go through most of the day without feeling sleepy. Their performance may not be up to speed; they may suffer other symptoms from insufficient sleep without being sleepy. Likewise, sleepy people may not be deficient of sleep, as sleepiness can have many causes. Also called drowsiness, sleepiness might be caused by sleep deficiency, but it might also be due to medications, circadian de-synchronization, and other causes.
Although the healthy rested brain is good about being firmly awake or firmly asleep, it is common for sleep to seep into waking and vice versa.
The yin-yang symbol gives some reflection of the interplay between waking and sleep. The light and dark sections hug each other, and there is a small circle of the other color in each. Nightime sleep, even those perceived as a good night’s sleep by the sleeper, are punctuated by microarousals or longer periods of waking. Similarly, we can have microsleep periods during the day, and occasionally naps. Is this why the American Academy of Sleep Medicine uses the yin-yang symbol?