Hypersomnia refers to either excessive sleepiness during the day or extended, overly long periods of nighttime sleep. Other words that are synonymous with hypersomnia are excessive daytime sleepiness (EDS), excessive sleepiness, or somnolence.
Hypersomnia is fairly common, with nearly 5% of the population affected. The age groups most affected by hypersomnia are teenagers and young adults. As with most sleep disorders, hypersomnia is underreported because many people inaccurately believe that always feeling sleepy and taking naps are normal behavior.
A distinction should be made between simple tiredness, fatigue, or depression, and hypersomnia. Sleep deprivation is a common state for most of the population - many people are tired or sluggish at certain periods during the day. The depressed person either will not want to get out of bed or will suffer from insomnia, even though he or she feels a nap will improve the depression. On the contrary, hypersomniacs are so sleepy during the day that they are compelled to sleep or "take a nap." These individuals fall asleep during movies, dinner, or even in conversation. Similarly, they may sleep ten hours or longer during one sleep cycle, rather than the average eight.