How many hours of sleep is appropriate and sufficient?

How much sleep is needed to ensure effective/fruitful consciousness and ability to work/study extra hours? Those who ever had to pull an all-nighter to get prepared for an exam or to meet a deadline would confirm on the irresistible inner demand to catch up with sleep shortly afterwards in any form, starting from a short snooze or catnap – wherever and whenever is possible/allowable, and ending up with 'watching' extended dreams on the following weekend. Although, the results of experiments (Roche Labs, 1966) showed, that much less of sleep time is needed to ‘repay the sleep debt’, compared to how much we had borrowed.

Consequently a dangerous temptation might occur to borrow more and more from the sleep time and repay less and/or apply a long grace period. In modern urban life with a rapidly changing political, economical, social, etc. environment our demand for additional time (over 24-hour) to achieve personal and career goals is increasing.

A vivid example of a person experiencing such a demand is a student who has got a full-time job and in parallel is studying in a solid university. Many ex-students recall a regular sleep time sacrifice as one of the challenges of their student life.

There are different opinions on the issue of minimum hours for sleep required in order to stay healthy, but what would be the optimum wake-to-sleep ratio for a long run for a person under constant time pressure?  Surprisingly the absolute duration of the sleep does not really matter. Instead, the sleep quality and passing full sleeping cycles are crucial.