They're common as you age and the tissues around your eyes weaken.

Why does it happen;
+ lack of sleep or stress
+ water retention or allergies
+ vitamin deficiencies
+ hormonal changes
+ aging
+ smoking
+ alcohol

General tips to help you;
+ Get enough sleep
+ Drink enough water
+ Reduce your salt intake
+ Try an eye cream with retinol or caffeine

Remember - You may notice puffy eyes for a number of temporary reasons. If it is constant, talk to your doctor about it.