Although many people state that the prefer their own company, the solitude or joy of being alone is not the same as the pain of feeling alone, which can lead to depression and low self-esteem, he said.

Chronic loneliness is linked with higher levels of the stress hormone cortisol in the morning, which raises the risk of heart attacks and strokes. Loneliness is also linked with higher blood pressure and a weakening of the immune system, Professor Cacioppo said.

One possible explanation for its link with ill-health is that loneliness seems to make people sleep less deeply. Lonely people tend to suffer from brief "microawakenings" in the night, which may reflect a nervousness about being alone at night, he said.