Typically, disorders of sleep cause disturbance either to the sufferer or to their bed partner. If total sleep time is reduced, this may lead to problems with excessive daytime sleepiness, which can affect work, driving, concentration, and relationships. ‘Sleepiness’ implies an intrusive desire to fall asleep, caused by some form of sleep deprivation or sedative drugs; this is different from ‘tiredness’, which implies general fatigue, lethargy, and exhaustion and is caused by a range of conditions, including depression, chronic disease, or a busy lifestyle. Adults sleep on average for 8 hours a night. Normal sleep consists of periods of deep or slow-wave sleep, interspersed with shorter periods of dreaming or rapid-eye-movement (REM) sleep. Periods of REM sleep lengthen towards the morning and hence some people remember their dreams on waking. Different disorders of sleep can affect any of these sleep stages.