From quick quizzes to full sleep studies, here’s how experts figure out why you’re so tired during the day.

Many people have days when they don’t feel full of energy. But if you regularly feel sleepy during the day, there may be an underlying reason. By figuring out what that cause is, you may be able to get help.

Most reasons for excessive daytime sleepiness are treatable. A doctor can perform medical tests to help you develop an action plan to address your sleepiness and achieve better health.

Read on to learn why you may have excessive daytime sleepiness, your options for testing, and what your doctor may want to know to make a diagnosis.

The Office of Disease Prevention and Health Promotion notes that daytime sleepiness can be a symptom of a sleep disorder.

Whether or not it’s a sign of a sleep disorder, excessive daytime sleepiness can interfere with daily living. You may regularly feel sleepy even after a good night’s rest. During the day, you might find it hard to concentrate, drive, or perform other routine activities.

A sleep disorder is just one potential cause of daytime sleepiness. It can also be secondary to another health condition or a sign of not getting enough hours of quality rest. Sometimes, it can have no identifiable cause.

Sleep disorders that may cause excessive daytime sleepiness include:

  • Insomnia: This is when you can’t get to sleep or stay asleep. Some people with insomnia also wake up very early and can’t fall back asleep.
  • Narcolepsy: Signs of this condition include sudden muscle weakness combined with excessive daytime sleepiness. You may have “sleep attacks” even while engaged in physical activity such as walking.
  • Restless leg syndrome: You may have aches or pains in the lower legs that get better if you move or kick the leg. Restless leg syndrome makes it harder to fall asleep and stay asleep.
  • Sleep apnea: In sleep apnea, you may periodically stop breathing. Your body regularly wakes up for you to take a breath, interrupting sleep.

A 2021 review outlined many potential causes for excessive daytime sleepiness. In addition to the above sleep disorders, the review also highlighted:

  • Idiopathic hypersomnia: With this condition, you feel sleepy even after you’ve had a good night’s rest.
  • Kleine-Levin syndrome: This rare condition causes episodes when a person sleeps up to 20 hours per day for a period of a few days up to a few weeks.
  • Circadian rhythm sleep-wake disorders: This is a group of conditions involving the circadian rhythm — the body’s internal clock that instructs when it’s time to sleep or be awake.
  • Sleepiness secondary to a medical condition: Infections, Parkinson’s disease, traumatic brain injury, and hypothyroidism are all conditions that may cause sleepiness during the day.
  • Sleepiness secondary to a mental health condition: Bipolar II disorder and atypical depression, among other mental health conditions, can also cause daytime sleepiness.
  • Sleepiness secondary to medication: Prescription medications (such as opioids) and substances (such as alcohol) can cause sleepiness.
  • Insufficient sleep syndrome: This can occur when you don’t get enough sleep at night because of everyday factors such as staying up later than usual. The result is a “sleep debt” that causes daytime sleepiness.

If you experience excessive daytime sleepiness frequently, you may want to speak with a doctor or sleep specialist.

Other reasons to consult a healthcare professional include:

  • Daytime sleepiness harms your ability to function, such as your ability to concentrate or perform routine tasks.
  • You have issues with memory, learning, or decision making.
  • You have a slower reaction time, especially during physical tasks such as driving.
  • You regularly feel sleepy, even after a full night’s rest.
  • You have issues falling or staying asleep, or you consistently wake up early.
  • You snore, stop breathing, or gasp while sleeping.
  • You have another medical condition or take medication that may cause sleepiness.

If a doctor or sleep specialist suspects you have excessive daytime sleepiness due to narcolepsy or idiopathic hypersomnia, they may recommend a multiple sleep latency test (MSLT).

The MSLT occurs after you have taken part in a sleep study, which measures how long and how well you sleep.

The MSLT consists of five naps, scheduled 2 hours apart, over the course of a day. At each scheduled nap time, you try to go to sleep, and the sleep technician records how long it takes for you to fall asleep.

Another tool to measure sleepiness is the Epworth sleepiness scale (ESS), a simple questionnaire you fill out to tell how likely you are to fall asleep in different situations.

Your doctor may have questions about how your sleepiness affects you in your daily life. To find a cause, they may ask in-depth questions about your medical history.

Some factors to consider telling your doctor or a sleep specialist include the following:

  • sleep symptoms, such as:
    • snoring
    • trouble falling asleep
    • daytime sleepiness
  • other symptoms that may be related to sleep, such as:
    • pain
    • depression
    • feeling run-down or low energy
  • sleep history, which you can record on a sleep log or sleep diary
  • other medical conditions that may affect sleep, such as hypothyroidism
  • medications, including prescription and over-the-counter drugs
  • drug or alcohol use, either recreational or prescription
  • changes in work schedules or other life circumstances that may affect sleep

Your doctor can use this information to help narrow down the potential cause of your daytime sleepiness.

Excessive daytime sleepiness can have many possible causes.

Consider speaking with a doctor or sleep specialist if excessive daytime sleepiness affects your daily life and ability to function. They can help diagnose an underlying cause by reviewing your medical history, asking about your sleep habits, and performing certain tests.