By the time people reach middle age, more sleep during the day such as an afternoon nap helps the memory and protects against its decline as long as they night-time sleep is not skipped, a study has found.Sound sleep in young and middle-aged people helps memory and learning.

\"As people grow older, they wake up more at night and have less deep sleep and dream sleep - both of which are important for overall brain functioning,\" said Michael K Scullin, director of Baylor University\'s sleep neuroscience and cognition laboratory.

If a person lives 85 years, he or she may sleep nearly 250,000 hours - more than 10,000 full days.

\"People sometimes disparage sleep as \'lost\' time. But sleeping well is linked to better mental health, improved cardiovascular health and fewer, less severe disorders and diseases of many kinds,\" Scullin noted.

For the study, researchers\' extensive review began with studies as long ago as 1967, including more than approximately 200 studies measuring sleep and mental functioning.

Participants aged 18 to 29 were categorised as young; aged 30 to 60 as middle-aged; and older than 60 as old.

Participants were asked how many hours they typically slept, how long it took them to go to sleep, how often they woke up in the middle of the night and how sleepy they felt during the day.

The research also correlated results from numerous brain-wave studies and experiments dealing with sleep deprivation, napping and sleep intervention such as sleep medications.

The article appeared in the journal Perspectives on Psychological Science.

\"It is the difference between investing up front rather than trying to compensate later,\" Scullin concluded.

Day napping in middle age boosts memory