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The Northwestern University researcher found when older adults go to bed lonely, sad or overwhelmed, they have elevated levels of cortisol shortly after awaking the next morning.

Elevated levels of cortisol -- a stress hormone linked with depression, obesity and other health problems when chronic -- actually cue the body on a day-to-day basis that it is time to rev up to deal with negative experiences, according to Northwestern University's Emma Adam, the lead investigator of the study.

"You've gone to bed with loneliness, sadness, feelings of being overwhelmed, then along comes a boost of hormones in the morning to give you the energy you need to meet the demands of the day", said Adam, an assistant professor of education and social policy.

Adam said her study, the first of its type, shows a sensitive day-to-day dance between experience and cortisol. Experience influences stress hormones, and stress hormones influence experience, she said.

The research appears online in the Proceedings of the National Academy of Sciences.