One simple secret to keep cool when it's too hot to sleep. Cheap too!

Turn your hot water bottle into a cold water bottle. Fill it at the cold tap and climb into bed. Tuck it somewhere like the back of your neck where there's good blood flow.  Even on the hottest of nights, in ten minutes your skin will be dry, you'll have cooled down and will probably be off counting sheep too.

Just remember to screw the plug tight,
dry the outside
and dry carefully the little space around the plug.

When this isn't an option.