Your body appreciates a bit of consistency.
One of humanity’s key abilities is its adaptability. As long as we can communicate and harvest resources, we can logic our way out of most problems. The human body is the same way; with a little bit of practice and exposure, it can optimize itself for most adverse situations, which is how you can adapt to changes in your lifestyle. However, one of the keys to this ability is consistency. If you don’t maintain some semblance of regularity, your body will be in a constant state of adaptation, meaning it’ll never reach optimization.
Sleep schedules are one of the best examples of this. I always go to bed and wake up around the same times, but I have a lot of friends who stay up into the wee hours playing games and whatnot, then sleep for an inordinately long time. Now, if there was a consistent schedule to this like, say, up til 4 AM, sleep til 1 PM, that’d be adaptable (if not especially responsible). But several of my friends just stay up until they’re bored and then sleep for a completely random amount of time. Unsurprisingly, they frequently complain about being tired. Because there’s no consistency to their sleep schedule, their circadian rhythms are completely out of whack and their bodies can’t figure out when they’re supposed to be tired.
Meal times are another good example. I usually get hungry for lunch around noon every day. Because I always eat lunch around then, my body starts anticipating when food is on the way, and gets ready to focus on digesting nutrients for a few minutes. If you don’t stop and eat three square meals a day, though, instead just peppering it with random snacks, your body won’t know when it’s time to start digesting. This can lead to unnecessary weight gain.
Your life doesn’t need to be 100% regimented, but if you want your bodily functions to perform the way they’re supposed to, you need to meet them halfway. Stick to a schedule, and your body can focus on optimizing itself.