As anyone who has unwittingly drifted off at their desk will know - tiredness can really creep up on you when you least need it.
But a new study is offering some good news: it claims to prove that we can bank sleep - and store it up in advance of a tiring event.Researchers in the US invited a group of volunteers to adjust their sleep patterns so the effect could be monitored.
For a week, one half of the group were allowed extra sleep, while the others were told to keep their sleeping patterns the same.
Then, the following week, they were all sleep-deprived.
"After this week of either extended or habitual sleep per night, all the volunteers came to the lab and they were given three hours of sleep, per night, for a week," explained Tracy Rupp from the Walter Reed Army Institute of Research.
They were then given tasks of varying complication, and tests showed that those who had banked the sleep were "more resilient during the sleep restriction".
"They showed less performance deterioration with regards to reaction time and alertness than the group that had been given the habitual prior sleep," said Ms Rupp.
The tests even showed that a week after the experiment the pre-stored sleepers were recovering better from their sleep deprivation than the habitual sleepers.
"What we're basically saying is if you fill up your reserves and pay back your sleep debt ahead of time, you're better equipped to deal with the sleep loss challenge."
For business people, the research suggests an end to the end-of-the-day slump. But for the military, it could be a life saver.
The researchers say that after some more testing the theory could be worked into pre-tour routines for the forces.
"Hopefully the operational planning could take this information into account and give soldiers additional sleep prior to a long term mission," says Ms Rupp.
The next step of research, they say, is to find out exactly why the extra sleep can be banked.
"We'd like to know what's going on psychologically in the brain.
"Next steps will be to do this sort of study but using brain imaging to see exactly what's going on in the brain."
Unfortunately, for now at least, the research team is unable to determine quite how much time you need to bank for maximum effect.
"My hunch would be that it's very individualised.
"Further research does need to be done to determine exactly how many nights are necessary."
And, as anyone who has tried to get an early night before a big day will know, it's one thing being in bed - it's another thing actually nodding off.
"A lot of our volunteers came into the lab and, knowing they had to go to bed at 9pm, were concerned that they would not be able to fall asleep."
"One way you might be able to capitalize and get some extra sleep is napping.
"Although this study didn't use napping, it has been shown to be very effective, at least improving performance in the short term."
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