One of my friends was just like this! She had a blood test, and they worked out she was ever so slightly anaemic which is what was causing the drowsiness. She has more recently been put on iron tablets and changed her diet and is much more aware of things. It might not be the same cause, but think about your diet. Tiredness can be caused by not eating the right foods such as fruit and vegetables, and eating too many fatty fast-food type meals. Make sure you drink plenty as this can make you feel tired as well.
Also the late nights repeatedly really won't be helping! I don't think I need to suggest how you could help yourself there!
If checking your diet doesn't help then it might be an idea to request a blood test and talk to your doctor about any possible causes. Rather than just listening to what they list, ask specifically for them to list to you what can cause drowsiness as you have described it, even if it may not be related to you, and you might hear something that you've changed over the past year or so that you didn't realise had affected you in that way.