According to UK charity SADA, these are the symptoms you should watch out for:
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Depression: negative thoughts, feelings of guilt and apathy that are out of character.
Sleep: sleeping more, including napping during the day, and/or early morning waking. Feeling exhausted all of the time, even when you’ve just woken up.
Eating: Can’t resist that jumbo jacket potato? Excessive cravings for carbohydrates like crisps or sweet foods like chocolate.
Mind: Memory problems and difficultly understanding people
Mood: Did you cry because you missed the bus? Feeling irritable or anxious for no apparent reason, low libido and wanting to be alone.
Illness: more colds/flu, a cracking headache even morphine couldn’t shift, and unusually bad PMT symptoms.