The body perceives the so called wet bulb temperature which is a combination of dry bulb temperature (what your thermometer reads) and relative humidity.
Air normally contains a certain amount of water vapor, but its capacity is fixed at each temperature point. When the temperature drops, the holding capacity of air drops, too, so even if absolute humidity (the amount of water vapor “dissolved” in air) is constant, the relative humidity goes up.
Above 50% relative humidity, evaporation is increasingly impaired. Evaporation is one of the main cooling mechanisms of the human body, as it’s one of the most efficient ways of cooling down stuff (for this reason it is used by air conditioners, fridges and freezers and so on).
Perspiration begins to appear, and if the humidity is high enough, evaporative cooling is completely shut down and the body must cool using other methods (I’m writing this post with my feet in a tub of cold water; or at least it used to be cold).
Depending on what kind of building you live into, the walls, ceilings and floors can also warm up during the day, due to a combination of direct heating by solar radiation and the outside air getting hotter, too.
So the building can follow a sort of out-of-phase cycle that make the environment hotter during the night (during which it may have a chance of cooling, usually partially). Hot surfaces not only heat up air inside the building, but also your body (as well as furniture and everything else inside) by emitting infrared radiation.
- If absolute humidity is constant during the day (assumption that may not hold true in your climate), then relative humidity increases.
- High relative humidity feels hotter because you can’t cool down effectively, or as effectively as in drier/cooler climates.
- Buildings can also release heat during the night
PS: heat transfer also depends on the difference between temperatures (in this case, the difference between the body’s and the environment’s); as the environment gets closer and closer to the body’s core temperature (~37 dC when awake, lower when sleeping), this also affects the ability of the human body to cool down.
If the internal heat production of the body outstrips the cooling capacity, the core temperature will begin to rise, potentially leading to a heat stroke, a condition that can be fatal.