However, if you are suffering any of the physical symptoms it should be checked out by your GP thoroughly as there may be other causes.
Depression can come on gradually, so it can be difficult to notice something is wrong. Many people continue to try to cope with their symptoms without realising they are ill. It can take a friend or family member to suggest something is wrong.
Doctor’s measure depression on different levels, mild or low in mood to moderate and severe. There is also psychotic depression; with psychotic depression you would experience hallucinations and delusions and would require hospital treatment. Doctors measure your level of depression through a questionnaire which is based on how you have felt in the last few weeks, and focuses on how much you have enjoyed living on a day to day basis. From this questionnaire they calculate a score that informs them of your level of depression.
Other forms of depression that would be included under this heading are post-natal/postpartum depression and manic depression/bipolar as it known. We must also mention S.A.D or Seasonally Affected Disorder, however with S.A.D doctors prefer to treat this with alternative forms rather than medication as it is believed a lack of vitamin E or lack of available sunshine in the winter months is the major cause of this type of depression.

Manic depression/bipolar
This form of depression is diagnosed when a person experiences episodes of extreme highs and deep lows; the highs show themselves as immense happiness, focus and joy. With these highs sufferers may experience insomnia or as mania. The lows are experienced as deep depressions while in this state, sufferers may experience life as very negative, be unable to stop crying. There is no known cause for this form of depression but it has a deep effect on the sufferer and their family as without the correct mood stabilizers this form of depression can be difficult to cope with. As with depression in general, bipolar has many levels of severity hence why this form should be diagnosed by a clinician before seeking any kind of therapy. However it has been shown that manic depressives can benefit from long term psychotherapy.

Post natal Depression/ postpartum
This form of depression effects women after child birth, however this is not to be confused with baby blues, a period of time usually 1-2 weeks where the mother may feel tearful and exhausted as the pregnancy hormone levels in the body adjust back to normal ranges. Post natal depression will show the same symptoms as depression, however there may be anxiety around caring for the child or about coping and been seen as a bad mother. As with depression, post natal can be measured on mild to moderate and severe level. In my own opinion I believe not feeling that sense of joy at a new-born only compounds feeing like a bad mother hence making the depression feel worse.

Other considerations
I would also like to mention that most people who suffer with depression can also suffer with anxiety. In general 10% of people who suffer with depression also suffer anxiety.
It may have a very confusing state of mind as you feel the apathy of the depression but hyper-activity of the anxiety this mix of sensations can cause sufferers to have anxiety/panic attacks.
Although no one knows exactly why, depression and anxiety often occur together. In one study, 85% of those with major depression were also diagnosed with Generalised Anxiety Disorder (GAD) and 35% had symptoms of panic disorder. Other anxiety disorders include Obsessive-Compulsive Disorder and post-traumatic stress disorder (PTSD). Because they so often go hand in hand, anxiety and depression are considered the fraternal twins of mood disorders.