Depression -

The causes of depression are not fully understood, but scientists think that an imbalance in the brain's signaling chemicals may be responsible for the condition in many patients.

Irritability, agitation or restlessness
Lower sex drive
Inability to focus, concentrate or make decisions
Insomnia or sleeping too much
Change in appetite and/or weight, eating too much or too little
Tiredness and lack of energy
Unexplainable crying spells
Unexplainable physical symptoms such as headaches or body aches
Feeling hopeless or worthless
Withdrawal from social situations and normal activities
Thoughts of death or suicide