What is drowsiness? Drowsiness refers to feeling sleepy or tired, or being unable to keep your eyes open. Drowsiness, also called excess sleepiness, can be accompanied by lethargy, weakness, and lack of mental agility. While most people feel drowsy at some point or another, persistent sleepiness or fatigue, especially at inappropriate times, can indicate a sleep disorder or other medical problem. Sleep-wake disorders are a common cause of drowsiness. These include a number of conditions, of which sleep apnea, insomnia, and narcolepsy are the most well known. Sleep apnea is a condition in which you stop breathing in the middle of the night, often several times a night. The deprivation of oxygen, as well as the constant sleep disruption, leads to constant daytime sleepiness, lethargy, and fatigue. It is also associated with more-serious conditions, such as heart disease, diabetes, and obesity. Narcolepsy is a condition characterized by daytime sleep attacks, and insomnia is the perception that you are not getting enough sleep or that the sleep you are getting is poor. Circadian rhythm sleep disorder occurs frequently in shift workers with fluctuating schedules and the resultant drowsiness can seriously impact family life, job performance and workplace safety. Depression, stress, and grief are also associated with compromised sleep. Treating or minimizing these conditions can greatly improve the ability to fall asleep and stay asleep. Certain medications are known to cause sleepiness. These include sedatives and tranquilizers, pain medications, and allergy medications, such as antihistamines. A sudden onset of drowsiness can indicate a life-threatening condition, especially if it is related to a head injury, exposure to extreme cold, or a medication overdose. Seek immediate medical care (call 911) if you, or someone you are with, have experienced a head injury, have taken too much medication or a new medication, or are suffering cold exposure (hypothermia) and become extremely drowsy.