Discussion of warming usually focuses on peak daytime temperatures. For a great many organisms the more consequential change is that nights are no longer getting cold enough.
Nights are warming faster than days
Increased cloud cover, humidity and greenhouse gas concentrations all reduce the rate at which the surface loses heat after sunset.
Daytime maximums are influenced by incoming solar radiation, which has not changed comparably, so the gap between daily maximum and minimum has narrowed.
The result is that the coldest part of the day has risen more than the hottest part, in most regions where long records exist.
Recovery from heat happens at night
Many animals tolerate high daytime temperatures by shedding accumulated heat overnight, and the strategy only works if the night is meaningfully cooler.
Warm nights mean an animal begins the following day with a heat load already present, and the deficit compounds across consecutive nights during a heatwave.
This is why extended warm spells cause mortality that single hot days do not, and why night temperature is often the better predictor of heat-related deaths.
Plants respire through the night
Plants photosynthesise during the day and respire continuously, and respiration accelerates with temperature while photosynthesis does not increase at night.
Warm nights therefore burn through more of the day's stored sugars, reducing the net carbon available for growth, fruiting and seed production.
The effect is well documented in crop yields and applies equally to wild vegetation, where it reduces the food supply available to everything feeding on it.
Cold nights perform ecological functions of their own
Frost and low night temperatures limit the range of many insects, including agricultural pests and disease vectors, by killing overwintering stages.
Milder nights allow those populations to survive further north and at higher elevations, and to complete additional generations within a season.
Many plants also require accumulated cold to break dormancy and flower properly, so insufficient winter chilling disrupts their timing independently of spring warmth.
Nocturnal activity windows are narrowing
Species that avoid daytime heat by being active at night depend on a period that is both dark and cool, and warming compresses the second condition.
In hot regions, some animals are already restricted to a few hours before dawn, which limits foraging time and therefore how much energy they can gather.
This constraint operates well before any lethal temperature is reached, which is why heat can reduce populations in places where nothing appears to have died of it.