A good night's sleep after a period of learning help brain preserve the most important memories, a news study has found.

Using data from a group of 44 college students aged 18 to 22, the study findings showed that sleeping helps brain use selective process to store most relevant information as long as four months.

The findings were presented Thursday at the Associated Professional Sleep Societies annual meeting, in Seattle.

The study was conducted by researchers at Harvard Medical School in Boston.

Think of sleep as a period of memory consolidation, where the sleeping brain calculates what is most important about a memory and selects the best candidates for long-term memory, said study author Jessica Payne, a postdoctoral fellow in psychology.

"It may be that the chemical and physiological aspects of sleep underlying memory consolidation are more effective if a particular memory is 'tagged' shortly prior to sleeping," she said.

She added that sleep seems to selectively preserve memories that are emotionally important and relevant to future goals.

People awakened after sleeping more than a few minutes are usually unable to recall the last few minutes before they fell asleep, earlier studies showed.

This sleep-related form of amnesia is the reason people often forget telephone calls or conversations they've had in the middle of the night. It also explains why people often do not remember their alarms ringing in the morning if they go right back to sleep after turning them off.

Source: Xinhua