The word "Depression" is derived from the latin "deprimere", which means to press down or deactivate, therefore it can be both mental and physical. Depression can have many different symptoms. In medicine one talks about "depressive syndrome", which is a summation of many different illnesses.
In persons with strong depressive conditions the function of the neuro transmitters is changed. Neuro transmitters are body produced chemical agents involved in cell to cell communication of nerve impulses. With depressive persons the neuro transmitters noradrelin and serotonin are low. Usually one administers medication to raise the level of these neuro transmitters. Newer research has found, that it might not be the lack of proper neuro transmitters, but the balance between them.
Latest research has also found, that a virus might be involved. People with depressions show the presence of a virus in their blood during times of severe illness.
Sleep is important for our well-being and vice versa. Not enough sleep can lead to depressions, depressions can lead to sleep disorders. Sleep disorders of this kind can only be cured if attacked at their source.