Key symptoms are:
- a persistent feeling of sadness and low mood
- loss of interest in the world around you and no longer enjoying things that used to give pleasure
- lack of energy and feeling tired all the time
Other symptoms can include:
- disturbed sleep, such as not being able to fall asleep during the night and then being sleepy during the day
- difficulties with concentration and making decisions
- low self-confidence
- poor appetite or an increase in appetite/"comfort eating"
- feeling very agitated or alternatively very apathetic
More top news
The terror attack carried out by Khalid Masood at Westminster that left four people dead lasted less than two minutes say police.
Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.
The family of the police officer who was murdered in the Westminster attack have thanked those who were with him 'in his last moments'.