With a lot of companies (e.g. Bangkok Busline, Siam FirstNew Viriya, etc.) serving the route and departures throughout the day you have plenty of options. Most buses leave after 8pm arriving early in the morning the next day. Note that in peak seasons (i.e. during Thai festivals like Songkran in April or Khao/Ok Phansa in July/October etc.) it is wise to book in advance. Prices vary depending on the level of comfort of the bus. If you can afford paying some extra baht, opt for VIP coaches with 24 seats (from 800 THB) which ensure enough space both for your legs and elbows. But even the cheapest buses from Bangkok (from 500 THB) are comfortable enough to sleep through the whole journey.