When the night does not cool down, the body sleeps differently. That asks for some adjustment.
During the day
By day, keep windows and curtains closed as long as it is warmer outside than in. Do not exercise just before bed. Drink enough water. Eat light in the evening, digesting heavy meals produces extra warmth.
Just before bed
Just before bed, take a lukewarm shower. Not cold, a cold shower actually makes the body produce more heat. It is counterproductive.
In bed
Sleep as low as possible, for warmth rises. Sleep naked, or in thin cotton pyjamas.
The right down
Choose a summer down, a lighter 100% goose down filling that regulates warmth and moisture especially well. Cotton bedding lets the skin breathe, synthetic does not.
Cooling down
Want to cool off? Run lukewarm water over your wrists, arms and ankles. These are the parts of the body that warm up fastest.