For a healthy person, it is not possible to get too much sleep. Sleep is controlled by the brain under the influence of a chemical called adenosine and the circadian rhythm. Adenosine builds up during wakefulness and causes sleepiness. The level of adenosine is reduced in the brain when a body has had enough sleep, and this will cause the person to wake up. If a person is sleeping more than normal or feeling excessively sleepy, there may be an underlying sleep disorder.
 

Dr Ignatius Mark is an ENT surgeon and director of sleep services at Ascent Ear Nose Throat Specialist Group, Mount Elizabeth Medical Centre in Singapore