The old adage "Don't go to bed angry" has a kernel of truth behind it. Anger isn't something you can sleep off -- the stress of an argument or a bad day actually stays in your body, researchers suggest.
When older adults go to bed feeling lonely, sad or overwhelmed, they wake up with elevated levels of the stress hormone cortisol in the blood, according to a study published today in the Proceedings of the National Academy of Sciences.
This is the first study to document how day-to-day experiences can affect stress hormone levels in the brain. Experience influences stress hormones, and stress hormones influence experience, the study's findings suggest.
Scientists already believe that chronically high cortisol levels can cause certain physical or emotional illnesses. Now it seems that high cortisol levels on one day just carry over to the next day.
Scientists looked at 156 older adults -- between the ages of 54 and 71 -- to see whether cortisol levels during a single day reflected feelings from the night before, or if those hormone levels would affect a day's behavior. Over three days, volunteers gave small saliva samples three times a day and described their feelings every night in a diary.
Adults who reported feeling lonely at night were more likely to have higher cortisol levels the next morning, meaning that a few hours' rest did not wash away any stress.
But the study's authors suggest a little extra stress hormone isn't necessarily a bad thing. That stress could prepare our bodies for the day ahead.
"Elevated levels of cortisol actually cue the body that it is time to rev up to deal with loneliness and other negative experiences," says lead study author Emma Adam, assistant professor of education and social policy and a faculty fellow at the Institute for Policy Research at Northwestern University.
In other words, older adults who go to bed feeling battle-weary, wake up with their stress hormones high and ready to fight.
"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," says Adam.
Over time, however, elevated cortisol levels actually wear the brain and the body out. "Cortisol levels often reflect the emotions people are experiencing," says Dr. Bruce Rabin, a professor of pathology and psychiatry, and medical director at the University of Pittsburgh Medical Center Healthy Lifestyle Program
"When our cortisol levels are too high, that can affect both our mental and physical health."
For example, if cortisol levels are always high, the parts of the brain that normally handle stress can wear out, so the body can't react to stress the way it used to. That's not a good thing, experts say, because our survival depends on our ability to recognize and react to stressful events.
"Cortisol helps us respond to stressful experiences and do something about them," says Adam.
"It is necessary for survival -- fluctuations in this hormone assist us in meeting the changing demands we face in our daily lives."
The good news is, there are a lot of ways to deal with the demands of daily life that will keep our brains calm instead of wearing them down, such as "having friends, being optimistic, being physically fit -- just walking, having a sense of humor," says Rabin.
Experts say taking a deep breath at night, or whenever you feel stressed, can help you have a calm night and a better day tomorrow.