- Sleep disorders: kind of problems of sleeping which include difficulties for falling asleep or staying asleep, falling asleep at inappropriate moments, be very sleepy, abnormal behaviours during sleep.
- Sleeplessness: difficulties for falling asleep and staying asleep. This it can be during two weeks, it can appear and disappear or it can be permanent (chronic).
- Excessive daytime sleepiness: difficulties for staying awake. People who have this disorder feel tired during the day. Symptoms which are not caused by sleeplessness are called hypersomnia.
- Circadian rhythm:s any biological process that displays an endogenous, entrainable oscillation of about 24 hours.
- Chronic: a condition or disease that is persistent or otherwise long-lasting in its effects.
- Syndrome: a disease or disorder that involves a particular group of signs and symptoms.
- Symptoms: a change in the body or mind which indicates that a disease is present.
- REM phase (Rapid Eye Movement): period of sleep during which dreaming takes place, characterized by rapid periodic twitching movements of the eye muscles and other physiological changes, such as accelerated respiration and heart rate, increased brain activity, and muscle relaxation.
- Insomnia: chronic inability to fall asleep or remain asleep for an adequate length of time.