Insomnia is defined as the inability to fall asleep, remain asleep, or get the amount of sleep an individual needs to wake up feeling rested. Its symptoms include difficulty falling asleep, frequent wake-ups during the night, waking up too early in the morning, daytime sleepiness, difficulty concentrating, and irritability. Insomnia can be acute (lasting one to several nights) or chronic (lasting from a month to years). It’s also the most common sleep complaint among Americans (especially women).