I used a bus to go from Dresden to Wien (and back) overnight a few years ago. This experience convinced me to never ever do something like that again.

1. I absolutely could not sleep on a bus that makes frequent turns on country roads.

2. On the way back to Germany, we got stuck for 2 hours in a passport check, because customs found a Bulgarian family on the bus that had previously been extradited, and there was a long argument before they left the bus.

There are buses that travel at night, but they're not comfy. i.e. they are not designed to be comfortable night-mode travel. In general, distances are too small to consider overnight trips. Since there aren't many good roads in Myanmar, trips take forever, which makes overnight travel sensible.