There is no single chemical that can make a person sleep instantly. The process of feeling sleepy is triggered by chemicals in the brain, including neurotransmitters like adenosine, histamine, and serotonin. Adenosine, in particular, is involved in the process of feeling sleepy. It slowly builds up in the blood when a person is awake, making them drowsy. While a person sleeps, the chemical slowly dissipates. Melatonin is another chemical that makes a person feel drowsy and ready for bed. It is a hormone released by the pineal gland that increases in the evening and peaks in the middle of the night, signaling that it is time to sleep. Other sleep-promoting factors include nitric oxide, prostaglandin D2, and cytokines. The sleep-wake cycle is also influenced by exposure to light, which can make it difficult to fall asleep and return to sleep when awakened.