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.

Answered by /u/Hypermeme