sleep is very important.. when you are in a deep sleep that is when your body repairs and recovers at the cellular level. yes that means if you don't sleep enough you will gain less muscle and gain more fat
it will have long term effects on everything from your fitness goals, to your thinking skills, to mental health, to aging. also the bad news is although you can 'sleep in' on the weekends and make some sleep back, that strategy has been shown to not work long term. so you must get CONSISTENT SLEEP, not 4 hours one night.. 6 the next.. and then 12 on the weekends.
if you feel okay getting only a few hours a night of sleep you are likely bipolar of have some sort of mental health issue going on but on the inside it is very detrimental for your health.