Insomnia is difficulty falling asleep or staying asleep, even when given the chance to do so. People with insomnia can feel dissatisfied with their sleep and usually experience one or more of the following symptoms: difficulty falling asleep, waking up often during the night and having trouble going back to sleep, waking up early in the morning or feeling tired upon waking. Individuals with insomnia may also experience fatigue, low energy, difficulties with concentration and memory, irritability and mood disturbances, and decreased performance at work or school. Insomnia is characterized based on its duration and can be acute or chronic. Acute insomnia is brief and often happens because of life circumstances (for example, difficulty sleeping the night before a job interview or exam, or after receiving stressful or bad news). Whereas chronic insomnia is more prolonged and is characterized by disrupted sleep that occurs at least three nights per week and lasts at least three months. A prolonged experience of chronic insomnia can lead to cravings, mood disorders, memory problems, premature ageing and depleted adrenal glands.