Your body runs on a bit more complicated system than this. generally speaking 1 night with little sleep here and there will not throw out circadian rhythms.However constant varaition of sleeping times (Shift work, etc ) does throw these out!However, hormonal release will be hampered! as sleep is necessary to put your body in the correct state for these hormones to be optimised!

So, a bunch of things happen when you only get about 4 hours of sleep. Some may not happen to YOU, these are just the things that have been observed to happen in sleep studies of the general population.#1 - Much higher cortisol levels in the morning and all throughout the day.#2 - Reduced leptin levels (lower satiety).#3 - Increase ghrelin levels (higher hunger).#4 - Possibly not enough time for the body to slip into ketogenesis... depending on when you ate your last meal. (If you ate carbs in the middle of the night during the insomnia, then definitely no ketogenesis.)  This is one of the cornerstones of the fat-burning component of Carb Back-Loading.Basically, undersleeping makes you really stressed out, makes you more hungry and less able to be satisfied by the food that you do eat.  You won't build as much muscle (probably).All that said... you can still follow the regular carb back-loading structure and still be pretty much okay, no "safe" time, just whenever you normally would according to your training time.  I'd advise making sure you get lots of fat in the first part of your day, maybe more than usual, to help prevent muscle catabolism.