Chronic loneliness (as opposed to the normal loneliness everyone feels from time to time), is a serious, life-threatening condition. At least one study has empirically correlated it with an increased risk of cancer, especially for those who hide their loneliness from the outside world.[5] It is associated with increased risk of stroke and cardiovascular disease.[2] People who are socially isolated also report poor sleep quality and thus have diminished restorative processes.[6] Loneliness is also linked with depression, a risk factor for suicide.

Quote from Wikipedia. Article 'Loneliness'