Sometimes you drift off to sleep easily. Other times you toss and turn for hours before you slip into a restful sleep. After lunch you may be dragging. Later, your energy levels soar just in time for bed.
How and when you feel sleepy has to do with your sleep/wake cycles. These cycles are triggered by chemicals in the brain.
Chemicals called neurotransmitters send messages to different nerve cells in the brain. Nerve cells in the brainstem release neurotransmitters. These include norepinephrine, histamine, and serotonin. Neurotransmitters act on parts of the brain to keep it alert and working well while you are awake.
Other nerve cells stop the messages that tell you to stay awake. This causes you to feel sleepy. One chemical involved in that process is called adenosine. Caffeine promotes wakefulness by blocking the receptors to adenosine. Adenosine seems to work by slowly building up in your blood when you are awake. This makes you drowsy. While you sleep, the chemical slowly dissipates.
Two body processes control sleeping and waking periods. These are called sleep/wake homeostasis and the circadian biological clock.
With sleep/wake homeostasis, the longer you are awake, the greater your body senses the need to sleep. If this process alone was in control of your sleep/wake cycles, in theory you would have the most energy when you woke up in the morning. And you would be tired and ready for sleep at the end of the day.
But your circadian biological clock causes highs and lows of sleepiness and wakefulness throughout the day. Typically, most adults feel the sleepiest between 2 a.m. and 4 a.m., and also between 1 p.m. and 3 p.m. Getting plenty of regular sleep each night can help to balance out these sleepy lows.
Your body’s internal clock is controlled by an area of the brain called the SCN (suprachiasmatic nucleus). The SCN is located in the hypothalamus. The SCN is sensitive to signals of dark and light. The optic nerve in your eyes senses the morning light. Then the SCN triggers the release of cortisol and other hormones to help you wake up. But when darkness comes at night, the SCN sends messages to the pineal gland. This gland triggers the release of the chemical melatonin. Melatonin makes you feel sleepy and ready for bed.
Some neurotransmitters help your body recharge while you sleep. They can even help you remember things that you learned, heard, or saw while you were awake. The neurotransmitter acetylcholine is at its strongest both during REM (rapid eye movement) sleep and while you are awake. It seems to help your brain keep information gathered while you are awake. It then sets that information as you sleep. So if you study or learn new information in the hours before bed, "sleeping on it" can help you remember it.
Other neurotransmitters may work against you as you sleep. Abnormalities with the neurotransmitter dopamine may trigger sleep disorders such as restless legs syndrome.
Even losing just 1 hour of sleep over a few days can have an effect. It can lead to a decrease in performance, mood, and thinking. Getting regular, adequate amounts of sleep is important. It can help you feel awake and refreshed during the day. It can also help you feel relaxed and sleepy at night. This helps make you ready for a long, restful night of sleep.