Its a tropical heat, not dry like the Med or Egypt, but quite bearable on the islands with a nice pool and the sea to dip into. As mentioned there are different weather patterns within Thailand so while Phuket might not be so great weatherwise in our summer, the island of Koh Samui for example will have better weather - its also quite popular with a younger age group. As for sleeping, many hotels will have air-conditioning as standard so its not a problem. Also hotel standards are high.