It has happened to all of us. We go to bed lonely, sad or with feelings of being overwhelmed. While that's clearly a miserable way to end the day, a new study says it also appears that our bodies attempt to compensate for these negative feelings by pumping up levels of the stress hormone cortisol shortly after we wake up the next morning. That may seem like a double whammy since elevated levels of cortisol, when chronic, have been linked to depression, obesity and other health problems.

But it's more complicated than that, according to Northwestern University Emma Adam, lead investigator of the study, which was published online by the Proceedings of the National Academy of Sciences. Going to bed in a funk actually cues the body to rev up the hormones in order to deal with the loneliness experienced the previous night. The hormone system anticipates possible trouble and wants to be ready. "The boost of hormones in the morning gives you the energy you need to meet the demands of the day," says Adam. It helps you get out of bed and go out into the world.

Cortisol is often characterized in negative terms, explains Adam, because of evidence that long-term elevations could be potentially harmful to physical health. But in the short term the stress hormone is adaptive and helpful. "Cortisol helps us respond to stressful experiences and do something about them," she says. "It is necessary for survival—fluctuations in this hormone assist us in meeting the changing demands we face in our daily lives."

The study was based on data from 156 older adults who participated in the Chicago Health, Aging, and Social Relations Study at the University of Chicago. The cortisol levels of the study participants were measured from small samples of saliva provided three times a day for three consecutive days. Study participants reported their feelings each night in a diary and researchers then looked at whether cortisol levels on a particular day were predicted by experiences the day before or were predictive of experiences that same day.