I noticed something similar (see also: my comment about using magnesium and getting better sleep on the thread about deep sleep yesterday).
When I get ~5ish hours of sleep one night a week I am surprisingly more alert the next day but I also crash much harder by night. I cannot sustain multiple days of little sleep but one short night doesn't seem to have such a detrimental effect.
One of my teachers told us to make sure we’re getting a good amount of sleep two nights before an important event (exam, job interview, competition, you name it). One can easily cope with a single sleepless night, but not with two in a row.
When I get ~5ish hours of sleep one night a week I am surprisingly more alert the next day but I also crash much harder by night. I cannot sustain multiple days of little sleep but one short night doesn't seem to have such a detrimental effect.