Very nice bed! We made a mistake on the orientation during setup and had to reverse about 12 steps which was stressful- pay close attention to the pictures in the instructions! And I agree with other reviews that a XL twin would probably fill in underneath the shelves a bit more, but honestly it would b harder to change the sheets with having to pull the whole mattress out of the bed to do so. Our twin mattress is just fine! No visible gaps.