I'm so tired!

I'm a ftm with a 5 1/2 week old little boy. So far, I feel really lucky that he is a great sleeper. Some nights he sleeps 5 or 6 hour stretches. However, I feel soooo tired even if he sleeps through the night. Is anyone else struggling with this? I wasn't sure if it's normal. I expected to be tired, but kind of thought my body would adjust by now.