A group of changes characteristic of depression can be considered as changes to bodily functions or rhythms. These are known as biological or somatic symptoms and are notable particularly in the more severe forms of depression. They include changes in several bodily activities which in themselves can be quite distressing and disabling.

Sleep may be disrupted either by delay in falling asleep or by waking up early or by frequent repeated intervals of waking during the night. As a result the refreshing quality of sleep is lost.

There may also exist a physical loss of energy or stamina, distinguishable from the loss of motivation, enthusiasm or enjoyment which, of course, may be evident. Some may nevertheless persist with greater efforts without managing adequately. This lethargy can be apparent to others as listlessness and slowness in initiating activity, and in a reduction in the number of changes in bodily posture and facial expression.

The patient may find it difficult to concentrate and to attend to mental tasks, which may appear as forgetfulness or frequent mistakes.

Appetite for food might be reduced, food becomes tasteless and is ignored and, over a period of time weight loss appears; constipation and nausea may occur.

Interest in sexual activity is often diminished, both because of inability to show affection and because of physical disinclination. In men we may see impotence and in women irregularity or even cessation of periods.