As @Tahl_eN@lemmy.world said in another comment, there’s concentration and there’s flow (aka the zone).

Concentrating takes effort, is often tiring, and requires disipline to block out distractions. It can feel good to consentrate on a problem or task, give it all your mental energy, and achieve your goal. It can be a fragile state though. If a distraction does break through it can completely disrupt your thought processes, causing you to lose track of everything you had in mind, and effectively sending you back to square one. Practice helps avoid that, but concentration is inherently mentally taxing.

Flow is different. You will probably only reach it through concentration, and may not jnitally be aware of the transition, but you’ll know it afterwards. The complex becomes simple, stuctures untangle themselves at a thought, you feel mental clarity unlike any other time, everything you’d been struggling with becomes effortless, and time ceases to have any importance. It’s more like a trance or meditation than a normal mental state, and you can stay in that state until your body physically runs out of energy. I’ve ended up sitting at my desk for nearly 24 hours without rising, and without eating or drinking, utterly engrosed in the task at hand, not noticing the sun setting and rising again, and felt entirely calm and rested at the end of it.