There are two main types of buses: local and intercity. Local bus is not air conditioned. It stops occasionally at each station, compresses the passengers without limitation. Everything is not done quietly- That is so typical nature in Sri Lankan state.
Intercity buses are air-conditioned, with a limited amount of transported passengers. Most of them are express buses and almost those don’t take any passengers from the road. You should refrain from traveling at night because of the roads and lighting.