The fact that they don't care about sleep doesn't mean it won't affect them. Exhaustion is going to slow down your reaction times, and caffeine only helps so much. Unless whole guilds are progress raiding on amphetamines 24/7, time of day is going to be a factor.
That's hard to do without either setting the DPS/HPS requirements so high that no one outside of the top 100 guilds is going to be able to do it without farming normal for months, or make it so artificially difficult or random that anyone outside of the top 100 will just get frustrated and quit. Bosses are tuned to be fun for the majority of (heroic-level) raiders, not to be a suitable challenge for the top 0.1% of heroic raiders. I agree that the last couple of bosses should be really hard, but heroic Immerseus can't be tuned to be a brick wall just to make the race for world first more interesting.