Answer to the question “Do insects sleep at night?”
Some insects are night-active, with the nocturnal I would certainly not suspect no night lytime sleep.
above: joint resting of many Dragonfly Butterfly , in the same together (sleep?) -Position
In the non-active half of the day, whether at night or during the day, as well as during cold periods such as winter or cold nights/days, all insects should rest motionless for a few hours, which is certainly good to describe with “sleeping”.
However, insects are often also motionless in action to keep warm through muscle work. It is quite unclear to what extent they “switch off” their ganglia activity, i.e. their brain activity, or to keep them on a thrift flame in dream phases.
Meanwhile (March 2019), sleep research in bees has shown that sleep deprivation does not significantly affect their ability to remember new things (as in humans), but their memory of already affected memorized (sleep deprivation extinction but not acquisition memory in honeybees; PDF: http://learnmem.cshlp.org/conten…).
In any event, the finding that an animal species does not have sleep is generally questionable, since the proof of non-fulfilment is difficult. The distinction between “thinking” and “dream” is problematic in principle and minute sleep must never be excluded. The closing of the eyes during sleep is prevented by insects, e.g. due to construction.
Elephants have an extraordinarily low need for sleep of about 2 hours among mammals, which does not need to be taken in one piece.
Elephants have several sleep episodes of about 15 minutes each.
above: Sleep while lying down, untypical in elephants
Most dolphins divide their sleep into even shorter stages of about 2 minutes.
Their sleep is very difficult to detect during the continuous glide under water, especially since they often sleep ONLY with ONE half of the brain. This was only discovered in neurophysiological studies in Bassins at the University of Basel about 30 years ago (Ask a grown-up: where do dolphins sleep?).