Excessive daytime sleepiness (EDS) is characterized by persistent sleepiness and/or a general lack of energy, even after adequate night time sleep. It is the primary complaint of patients seen in sleep clinics, and affects up to 12% of the general population. Persons with EDS are compelled to nap repeatedly during the day; fighting off increasingly strong urges to sleep during inappropriate times such as while driving, while at work, during a meal, or in conversations. As the compulsion to sleep intensifies, the ability to competently complete tasks sharply diminishes, which often mimics the appearance of intoxication.

EDS can be a symptom of a number of factors and disorders:

• Insufficient quality or quantity of night time sleep.
• Misalignments of the body's circadian pacemaker with the environment (eg. jet lag, shift work or other circadian rhythm sleep disorders).
• The symptom of an underlying sleep disorder, such as narcolepsy, sleep apnea, hypersomnia or restless legs syndrome.
• Disorders such as clinical depression or atypical depression.
• Tumors, head trauma, anemia, kidney failure, hypothyroidism or an injury to the central nervous system.
• Smoking, drug or alcohol abuse.
• Genetic predisposition.

The average person requires 7 to 8 hours of sleep per night. If a person routinely gets less than this, it will most likely cause excessive daytime sleepiness. Another common problem can be sleeping in a noisy environment. This can cause many short awakenings, so brief that a person doesn't remember them the next morning. This results in non-restful sleep with resulting excessive daytime sleepiness. If a person gets 8 hours of sleep a night and still feels sleepy during the day, one should consider that they may have a medical problem which hinders their ability to obtain restful sleep.
A common medical problem that can cause increased daytime sleepiness results from having a breathing problem during sleep. Snoring for example, is a form of obstructed breathing.
Another common medical problem causing frequent disruptions of sleep is called Periodic Limb Movements of Sleep. These leg movements can fragment sleep, resulting in increased daytime sleepiness. Frequently people with this problem also have what is called Restless Legs Syndrome (RLS). RLS occurs when a person is sitting still or lying in bed, which makes it difficult to keep their legs still. This frequently can keep people awake, causing insomnia. People with this problem feel that moving their legs relieves this restless sensation.

The previous problems are all treatable. Unfortunately, many physicians are not yet familiar with treating these conditions. What is important to understand is that we need about 8 hours sleep per night to function properly, although some of us need more and others can get by with less. If there is something that causes many brief disruptions in your sleep, then your brain may not get the rest it needs. You may not be aware of these awakenings or disruptions because they are too brief to register in your memory.