Modafinil is a stimulant used by doctors to treat people with chronic sleeping problems such as narcolepsy - where the person can fall asleep at anytime and without warning.
It is used by the military in times of combat to keep soldiers awake for extended periods. Non-medical use is thought to be on the increase among shift workers such as nurses and pilots.
The drug though similar in some of its effects to Ritalin (used to treat attention deficit disorders in children) is not an amphetamine-related drug.
Its effects are to keep the user awake for prolonged periods, up to 40 hours. It reportedly lacks the agitation, irritability, nausea and comedown associated with amphetamines, though doctors do warn it can be addictive if regularly taken over a long period.
Doctors are not sure as to how the drug actually works, though it is thought to act on dopamine, a neuro-transmitter that affects mood and thought and keeps the person awake.
Like all drugs that keep you awake, long-term use can cause psychological and physical problems such as fatigue and disorientation - symptoms that can only be resolved through a good night's sleep.