Feeling sleepy usually comes from how your body’s internal clock, brain chemicals, habits, and health conditions all interact. Different factors can make you sleepy at normal times (like night) or at times when you would rather feel awake.
Body clock and hormones
The body has a circadian rhythm that makes you naturally feel more awake in the day and sleepier at night. Light and darkness help set this clock, and darkness triggers the release of melatonin, a hormone that makes you feel drowsy as it gets late.
If you get a lot of bright light at night (phones, screens, room lights), your clock can shift, and you may feel sleepy at odd times or struggle to fall asleep when you want.
Brain chemicals and sleep pressure
As you stay awake, a chemical called adenosine builds up in the brain and creates “sleep pressure,” making you feel increasingly sleepy the longer you have been awake.
Certain neurotransmitters help switch the brain from wakefulness to sleep, and when sleep‑promoting neurons become more active (and wake-promoting ones quiet down), drowsiness increases.
Lack of sleep and poor sleep
Not getting enough total sleep (sleep deprivation) is one of the most common reasons for feeling sleepy during the day.
Even if time in bed is long, frequent awakenings, pain, breathing problems, or conditions like insomnia or sleep apnea can fragment sleep so you wake up unrefreshed and feel drowsy later.
Lifestyle, food, and medications
An irregular schedule, shift work, stress, low physical activity, and an unhealthy diet can all contribute to tiredness and sleepiness.
Alcohol, sedatives, some pain medicines, antihistamines, and other drugs can directly cause drowsiness or disrupt sleep at night so you feel sleepy in the day.
Illnesses and mental health
Medical problems like sleep apnea, hypothyroidism, anemia, diabetes, infections, and chronic pain conditions can all lead to excessive daytime sleepiness.
Mental health conditions such as depression, anxiety, and stress can both disturb sleep and drain energy, so you feel more sleepy or fatigued.
