Temporal relations between unexplained fatigue and depression: longitudinal data from an international study in primary care.
Skapinakis, Petros, Lewis, Glyn, Mavreas, Venetsanos·Psychosomatic medicine·2004
This study tracked over 3,000 people from primary care clinics worldwide for one year to understand the relationship between unexplained fatigue and depression. They found that people with depression were four times more likely to develop fatigue, and people with fatigue were nearly three times more likely to develop depression. The results suggest these two conditions may increase the risk of each other developing.