If you don't sleep...

you will not learn well the day after...

but, you will still be able to remember bad things.

students deprived of their sleep did perform 40% worse in memory tests than students well rested.

but, and here's the kicker, they did perform only 19% worse in words with negative emotional charge, such as 'cancer' or 'jail', and 59% worse in good things such as 'cookies' and 'holidays'...

I do agree with the authors of the paper, who suggest an evolutionary safeguard mechanism to avoid forgetting threats.

this way, if something bad happened the day before, you still have a chance to avopid it... neat uh?