We are talking about wizards, sorcerers, clerics, and warlocks, who have either (1) spent years strengthening their minds to focus on and control magic powers until it becomes second natures, (2) were born with a natural talent for magic, or (3) have abilities bestowed upon them by a higher power (and probably some training/discipline involved as well).
I liken it to learning to sleep on a bed without rails. You need to be careful just sticking a young child on a high bed without rails. They are likely to roll right off of it. That's how I broke my collar bone when I was a kid. Once you are older that if very unlikely to occur. As an older kid, I took naps in trees, bunk beds with our railings, hammocks, etc.
Another example would be how it can be dangerous to have a non-parent adult share a bed with an infant but how incredibly rare it is for parents to roll over on their own infant.
A zero-level Wizard's apprentice would probably need to take great care and work with some aids to keep concentration on certain spells. But once they have the background that justifies them becoming 1st level adventurers, the spells they know are ingrained. I would take a real jolt, like a smack to the head or truly surprising/shocking/dangerous/emotionally charged environmental conditions to break their concentration.
Another way to look at it is casting a spell that requires concentration is like engaging in self-hypnosis as part of the casting.
Scientists who study hypnosis are not exactly sure how it works, but they know that it helps change our expectations. Suggestions made during hypnosis help the mind take control over the body. "You subconscious has grabbed hold of that message, replaying it again and again." (from
WebMD, not exactly a peer-reviewed journal, but sufficient for making a simile).
So, I agree with other posters. You can maintain concentration for the spells duration, including during normal, self-guided sleep (as opposed to being physically or magically knocked unconscious or magically put to sleep).