Several chemicals are involved in the process of feeling sleepy. Here are some of the key chemicals that make you feel sleepy:

• Adenosine: This is a chemical produced by the brain that increases sleepiness. Adenosine seems to work by slowly building up in your blood when you are awake, making you drowsy. While you sleep, the chemical slowly dissipates.

• Melatonin: This is a hormone released by the pineal gland that helps you feel sleepy once the lights go down. The amount of melatonin in our bodies starts increasing in the evening and peaks in the middle of the night, letting us know it is time to sleep.

• GABA: This is a neurotransmitter that is associated with sleep, muscle relaxation, and sedation. It can help to switch off or dampen the activity of cells that signal arousal or relaxation, promoting sleep.

• Nitric oxide: This is a small gaseous molecule synthesized by enzymes in the brain that promotes sleep through a variety of mechanisms, including promoting the release of adenosine.

• Norepinephrine: This is a neurotransmitter that increases your brains level of arousal and helps contribute to wakefulness. However, it can also keep you from falling asleep easily.

Its worth noting that caffeine promotes wakefulness by blocking the receptors to adenosine. Additionally, exposure to light can make it difficult to fall asleep and return to sleep when awakened, as specialized cells in the retinas of your eyes process light and tell the brain whether it is day or night and can advance or delay our sleep-wake cycle.