Nerve cells that form a "sleep-switch" and are lost from the brain through the effects of ageing and disease lead to disturbed sleep in old age, scientists have discovered.
Andrew Lim, from the University of Toronto, studied the brains of 45 individuals who had enrolled previously in a community-based study called the "Rush Memory and Ageing Project".
These participants had been followed since they joined the study, in good health and aged 65, until the time of their death.
As part of the study, every two years they had undergone actigraphy measurements in which they wore movement sensors for a 7-10 day period, allowing researchers to probe periods of rest and sleep.
Individuals with the lowest numbers of nerve cells in a brain region called the intermediate nucleus, which is in the brain's hypothalamus, had significantly reduced levels of sleep compared with individuals with higher cell counts.
People with over 6000 surviving nerve cells in this area spent more than 50% of their rest time asleep compared with individuals with fewer than 3000 cells who spent less than 40% of their rest time asleep.
This region in the human brain corresponds to an area of the rat brain called the ventrolateral pre-optic nucleus, which scientists have shown previously to be critical for normal sleeping patterns. Ablating the nerve cells in this area, which communicate with each other using the nerve transmitter chemical galanin, leads to the animal equivalent of insomnia.
These findings suggest, say the researchers in their paper in the journal Brain, that this brain region is likely to operate as a "sleep switch" and loss of cells in this area might account for the sleep disturbance commonly reported in diseases like Alzheimer's.