Sleeping literally changes our very physiology. Our core body temperature drops which allows certain proteins to work differently than they do during our “waking temp,” as a broad example. It’s not something we’d want easy control over.
Most importantly the process of getting sleepy is highly regulated by not only our Circadian rhythm but also by other hormone systems.
We need to burn energy to feel fatigued (when we use ATP and make Adenosine as a byproduct, which signals fatigue in humans).
We need a lack of blue-wavelength light to initiate the process of releasing melatonin at night, which makes us sleepy and helps initiate the sleeping-end of our Circadian processes.
We don’t have voluntary control over sleep because it’s chemically regulated. Adenosine, melatonin, hypercretin (Orexin), etc..
It’s not something we can flex like a muscle. It’s essentially hormonal in nature and therefore requires us to use drugs (meaning ligands that bind to targets in our body) to control it.
Humans are strictly diurnal, meaning we are awake during the day and asleep at night. Cats are crepuscular which means they tend to be active around dusk and dawn and sleep in-between times. This means they don’t require a solid 8 hours sleep a night like we do, and can more easily nap throughout the day.
Explains why stress that produces adrenaline breaks this delicate chemical process? It’s so hard falling asleep when thinking about work, sick relatives, etc…
It might be related to it being evolutionarily advantageous. If one of our ancestors heard a predator nearby, for them to have a kick of adrenaline and climb to a higher branch would have perhaps made them more likely to survive than heavy sleepers that blew it off and had a saber toothed bear climb up the lower branches and snatch them.
So the genes that influenced you being able to have a kick of adrenaline that could keep you awake if you sensed danger got passed on.