Up to one-third of the world’s population reports dissatisfaction with their sleep. Amongst the elderly and women, this percentage would be on the higher end. As high as 50% of older adults report sleep disturbances and insomnia. Using the DSM-5 criteria, up to 10% to 15% of adults would be classified as having short-term insomnia. Women are two times as likely to have insomnia than age-matched men."

Source : StatPearls [Internet], 2023.

Many cases of insomnia do have a readily identifiable set of causes - e.g.including : psychological stress, grieving, shiftwork, chronic pain, heart problems, hyperthyroidism, heartburn, restless leg syndrome, menopause, internet addiction, medications, caffeine, nicotine, and alcohol, &etc.

A proportion of cases, however, don't have any obvious cause. The exact prevalence is unknown, but possibly <1% of the population (ref,). Put another way, that's up to 70 million people.

In such cases the disorder is termed as 'Primary Insomnia' or sometimes 'Idiopathic Insomnia', or 'Short Term Insomnia',

The exact mechanism causing short-term insomnia remains unknown, even though there are many proposed models. There is an emerging consensus that short-term insomnia is a disorder of hyperarousal.
Hyperarousal is a state of increased somatic, cortical, and cognitive activation. Measured in physiologic terms, this would mean that patients with short-term insomnia would demonstrate increases in cortisol levels, body temperature, 24-hour metabolic rate, and heart rate.
There is also a growing body of knowledge that links short-term insomnia to the upregulation of wake-promoting chemicals of orexin (also known as hypocretin), histamine, and catecholamines. The presence of these chemicals couples with the downregulation of the sleep-promoting chemicals adenosine, serotonin, melatonin, and GABA. More research is necessary regarding the precise molecular mechanism responsible for hyperarousal."

[ Source as above ]

Also see : Sleepplugin-autotooltip__plain plugin-autotooltip_bigSleep

"The purposes and mechanisms of sleep are only partially clear and are the subject of intense research"

Source Wikipedia"

Although extensive research in humans and animals has shown unequivocally that sleep is essential, the reasons why it's required are as yet unknown. Long term sleep deprivation not only severely impairs cognitive and motor skills, it's now been shown that it can cause physical damage to brain structure - and, in extreme cases, irreversible damage.