When individuals are chronically restricted of sleep for periods of 2-3 weeks, we instead find that cognitive impairment accumulates day by day, almost linearly. There is no sign of saturation or leveling off. Things just continue to get worse and worse. Paradoxically, delta waves do level off, just as the two-process model would predict, and so do subjective ratings of sleepiness, meaning people become less and less aware of their level of objective impairment as they are increasingly sleep restricted. After two weeks of getting 6 hours of sleep per night, individuals have the same reaction time as somebody who has been awake for 24 hours, which is approximately equivalent to an individual with a blood alcohol concentration of 0.10%. After two weeks of getting 4 hours of sleep per night, individuals have the same reaction time as somebody who has been awake for 48-72 hours.
The process of recovery also seems to be much slower after chronic sleep restriction, although it has not yet been well quantified. For chronic sleep restriction, there seems to be a much closer to one-to-one correspondence between hours of sleep lost and hours that must be paid back to return to baseline performance. Certainly, it is not possible to reverse the effects of chronic sleep restriction in a single weekend.
— Reddit: Ask Science - “Can a person ever really catch up on sleep?”, answer by computational neuroscientist whatthefat.