The most common forms of depression are characterized by a general lowering of the level of activity whether this is shown by a fall in appetite, a loss of sleep, or a reduction in activities.

Paradoxically a few depressed patients experience a reversal of this usual pattern and therefore find themselves sleeping more, though not generally feeling refreshed and energetic as a result, and eating more and consequently tending to gain weight. These patients' illnesses may be missed by doctors and the true significance of their symptoms not recognized, so that treatment may be delayed or withheld.