The primary cause of feeling tired or sleepy during the day is insufficient sleep. This problem occurs when you fail to get enough sleep each night. Most adults need 7 or more hours of sleep on a regular basis to feel alert during the day. Excessive daytime sleepiness is an essential feature of hypersomnias such as narcolepsy. Other sleep disorders such as obstructive sleep apnea also can cause you to feel tired during the day. Sleepiness can be a symptom of mood problems such as depression. Some medications also can cause you to feel tired. You should talk to your medical provider to identify the reason why you feel so tired.