Sleeping with a lonely heart brings a hormone jolt in the morning. That surge is delivered by cortisol, a stress hormone often linked with negative biological effects, such as depression and obesity. But in daily life, researchers studying older people have found, cortisol might be helpful in getting the forlorn out of bed.

Throughout the day, cortisol levels in the bloodstream vary. When people wake up in the morning, their cortisol is low. About half an hour later, though, it spikes upward. Then it dwindles, dropping to its lowest level around midnight. But, scientists say, the daily ups and downs of life might affect how much of this hormone is actually released.

Emma Adam, an associate professor of human development and social policy at Northwestern University, and her colleagues recruited people born between 1935 and 1952 from an ongoing health and aging study in Chicago. For three days participants swabbed their mouths to collect cortisol-containing saliva. They swabbed when they woke up, 30 minutes later and right before they went to sleep. Each night, the participants also filled out questionnaires that inquired about their day, asking them to note if they felt lonely, angry or tired. "The question of loneliness is particularly important for older adults," says Adam. Loneliness is a stressor that can lead to physical and health problems in older people.

Adam found that each person's daily experiences interacted with their cortisol levels. Older adults who went to bed lonely, sad or overwhelmed got a bigger jolt of cortisol the next morning than those who felt otherwise, the researchers report online in this week's Proceedings of the National Academy of Sciences USA. This boost, Adam says, might get lonely people up and out into the world where they see and speak with other people. The amount of cortisol in this burst, Adam hypothesizes, is regulated so that they have as much as they need to get through the day.

Other social and emotional experiences also influence this hormone, the researchers have found. Older adults who reported themselves as being angry had higher levels of cortisol throughout the day. Their hormone levels were revved up and did not slide off as the day progressed. Conversely, people who never got that pep-up shot of cortisol in the morning felt tired throughout the day. "Our biology and experience are much more tightly interwoven than you think," Adam explains. So lonely hearts need not stay lonely if their body can help it.