Actually when we go to bed we're kind of deciding to go to sleep. Some people take 5 mins, others close to an hour (me) but its a decision. Your thoughts, ego, are also not the same as the body. Maybe its some kind of negotiation. The body reads some signs then maybe tries to guess if its time to sleep. Other times its the other way around, the body dexidoes to do it regardless of what your mind wants.

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