Insomnia, or sleeplessness, is a sleep disorder in which there is an inability to fall asleep or to stay asleep as long as desired. nsomnia is most often thought of as both a medical sign and a symptom. Insomnia can be short term (up to three weeks) or long term (above 3–4 weeks), which can lead to memory problems, depression, irritability and an increased risk of heart disease and automobile related accidents.