Insomnia usually refers to a subjective experience of unsatisfactory sleep duration and/or quality that affects daytime social functioning. The common clinical forms of insomnia are: 1. prolonged sleep latency: sleep onset longer than 30 minutes; 2. sleep maintenance disorder: ≥2 awakenings at night or early awakening in the early morning; 3. decreased sleep quality: shallow sleep and dreaminess; 4. shortened total sleep time: usually less than 6 hours; 5. residual daytime effects: feeling dizzy the next morning, lack of energy, drowsiness, and weakness.