Most people function on a wake-sleep cycle that corresponds to our 24 hour day. The mechanism that controls that cycle is known as circadian rhythms. Anyone can have that cycle interfered with, resulting in sleep disturbances that are usually very temporary. Jet-lag is a good example of an event that can cause such a disruption; one that many of us have experienced. Getting over jet-lag is usually just a matter of letting your brain adjust to the new time zone. The day-night cycle is mostly responsible for that adjustment, responsible for resetting your circadian rhythms.
Alzheimer's disease and some other brain disorders can damage the part of the brain that regulates circadian rhythm. That makes it much more difficult to recover from disruptions to the normal wake-sleep cycle.
An increasing amount of evidence shows that bright, full spectrum light, on the magnitude of 10,000 LUX, can reset the circadian rhythm in people suffering from Alzheimer's. Daily exposure to this type of light helps dementia patients with sleep disorders sleep longer and spend more time in deep sleep. A study reported in the Journal of the American Medical Association reported that cognitive deterioration slowed with regular exposure to bright light, and symptoms of depression decreased.