Insulation only works if night-time air temps are significantly different than daytime temps - like in arid desert regions. Whole house fans are extremely effective under these conditions.

Say the night time temp is 16C and the daytime temp is 36C. Opening the windows at night and turning the fan on will cool the house down to 18-20C. Then closing everything up in the morning (windows and blinds) the house will only warm up to around 25-27C during the day. Hot but not deadly.

When the temps peak over 40C and the nighttime temps are 30C, people die without some type of active cooling. Especially if the humidity is high as well. Even a window unit that pulls the temp down to 25C at night can save lives.