There are whole bunch of reasons, but let's move from the worst variants.
1)Puffy eyes can be a significant signal of cardiovascular or renal systems disorder. But, actually, puffy eyes wouldn't be the main symptom. When the heart is the problem, the edema under your eyes appears mostly at night, in a case with kidneys - in the morning. In case you feel problems with your heratbeat, you actually feel the heart's bump - you need immediately visit your doctor.
2) Also, it can occur when you are tired and sleepy. Five additional hours in your bed must change this situation.
3) If you have weed/alcohol abuse - it's also significant symptom
4) Rare case, but also possible - it's how it works in your body. Genetics+conditions. Honestly, not the worst configuration lol.
To fight with the puffiness under your eyes try to use some night cremes for skin, you can always make a "cucumber mask". Don't limit your water consume!!! Drink as much as you want. If you feel that it doesn't work - try to take some diuretics like Spironolakton or some light ones, but BEFORE - talk with your doctor!
Cheers!