Loneliness could be bad for your health, psychologists in the US have warned.

University of Chicago psychologists Louise Hawkley and John Cacioppo studied a group of college-age individuals and adults aged 50 to 68 to reach the conclusion.

When the psychologists looked at the lives of the middle-aged and old people in their study, they found that although the lonely ones reported the same number of stressful life events, they identified more sources of chronic stress and recalled more childhood adversity.

When faced with similar challenges, the lonelier people appeared more helpless and threatened. And ironically, they were less apt to actively seek help when stressed out.

The psychologists took urine samples from both the lonely and the more contented volunteers, and found that the lonely ones had more of the hormone epinephrine flowing in their bodies.

Epinephrine is one of the body's "fight or flight" chemicals, and high levels indicate that lonely people go through life in a heightened state of arousal. As with blood pressure, this physiological toll likely becomes more apparent with aging.

Since the body's stress hormones are intricately involved in fighting inflammation and infection, it appears that loneliness contributes to the wear and tear of aging through this pathway as well.

When people experience stress, the bodies normally rely on restorative processes like sleep to shore up. But when the researchers monitored the younger volunteers' sleep, they found that the lonely nights were disturbed by many "micro awakenings."