REMEMBER: The law says you must hold and carry a valid medical certificate or medical declaration every time you fly and that it is your responsibility to ensure that you are fit to fly.
A large number of aviation accidents identify pilot fatigue or medical factors as contributory. Pilots often get away with flying in a less than optimal physical or mental condition, but it can have a critical impact in the event of an emergency or a more insidious issue that an alert pilot would stand a better chance of surviving.
'IM SAFE' is a common mnemonic for self assessing fitness before flight:
Illness - are you suffering from any?
Medication - are you taking any?
Stress - are you suffering from any?
Alcohol - when did you last drink?
Fatigue - are you well rested?
Eating - have you eaten recently?