Normally, a comfortable temperature is around 90º Fahrenheit, perhaps a little warmer in the winter. a little cooler in the summer. If you fill the waterbed with cold water allow 2- 3 days to heat. It's best to fill the waterbed mattress with warm water from your bathroom or kitchen facet ( all the way hot and cold at the same time ) to enjoy immediately. Never fill the mattress with hot water, this can stretch the vinyl .