Did you used to think you can "catch up" or "store up" on sleep? I did, until I was watching one of those news magazine shows several years ago and I was surprised to learn that once you miss a good night's sleep, you can't get it back. The damage (whatever it is) is done, and sleeping extra the next day or two doesn't give you a sleep deficit credit. And, as you probably learned from experience, sleeping extra tonight won't prevent you from being less tired the day after tomorrow if you don't get a normal night's sleep tomorrow. Following this? No matter if you are or aren't, I'm about to switch topics to something I'm much more of an expert on.
As for fitness, I'm a firm believer that you can treat exercise much more like a bank account than you can with sleep.
I'm watching this happen to myself right now. My mother just passed away in New Jersey, and I had to spend the last 10 days there eating bagels, cream cheese, lox, tuna salad, chopped white fish, creamed herring and more. If this sounds foreign to you, then you've never seen a kosher Shiva platter. (Kosher does not mean "healthy" or low-calorie, by the way.) Then, the rest of the gift baskets came from Godiva, Harry & David and the other fine purveyors of comfort food.
Of course when a tragedy you're not prepared for happens (is that redundant?), you go into a type of shock and everything is turned topsy-turvy, such as my typical fitness regimen. Basically I did zero exercise (aside from cleaning up for visitors and going through every box in the garage). Then I remembered. I had been working out almost every day since July Fourth weekend, and before that, almost every day since returning from a European cruise vacation in mid-May. That's a lot of stored up credit! I further comforted myself by remembering that aside from one major and one minor spine surgery this past decade, I've missed very few workout days. I put a lot of exercise into my proverbial fitness 401(k)!
An occasional quip I use in many of my Spinning classes is, "all the hard work you're doing now is like an investment you can draw on later." My own words rung true as I lived for more than 10 days on junk and comfort food, did no actual exercise and suffered thru mega-tons of emotional stress: a three-prong kryptonite bomb to the body!
I can't say I came home unscathed. I see my body every day and know when muscles are atrophying and when fat cells are growing, but at first glance in the mirror (from the neck down, anyway, because the face is a dead giveaway something's wrong), I still see most of my muscles.
Several studies have actually been done on training vs. detraining with athletes, but depending on how hard and how much you train, the type of workouts you do and how long you laze around for, results will vary. Basically, if you've been training like an athlete for a while, you lose your muscle tone and definition more slowly than if you have just started your fitness routine on your last birthday after 10 years of jockeying your La-Z-Boy.
However, it's never too late to start. Studies show you can build muscle well into your 90s. It may be an uphill battle as you age, so it really just comes down to what kind of soldier you are. Start building up your fitness bank account (safely, without injuring yourself, or it's all for naught) and when that unexpected emergency derails you, you'll have a cushion to soften the blow.
On a side note, the best solution is to just do a little bit of exercise at moderate intensity when you're taken out of your normal routine such as from extensive traveling or marathon work days. Even just once a week can help prevent steep fitness declines. If you break a limb or injure a part of your body, you can still exercise the rest of you. I work with injuries all the time (including my own) and there's no reason to languish. If you're suffering from an illness, then talk to your doctor, but often you'll find that even when you're sick, a little dose of exercise is some good, cheap medicine.