Everyone has a daily requirement for sleep. Your sleep must be good quality, that is, you must experience the right amount of each sleep stage, especially deep sleep and REM sleep and you should not experience frequent awakenings. Your sleep must also be of sufficient quantity, normally seven to eight hours [1].
If you significantly disrupt your sleep quantity or quality you may become fatigued during the day and your performance may suffer. I call this Acute Sleep Disruption. So what is significant? If you shorten your sleep by 30 minutes or more, or you mess up the quality of your sleep by changing the pattern of sleep stages, and it has happened within the last three days, that’s significant.
Awakenings, staying up later and getting up earlier can all disrupt the quantity of your sleep. If they reduce the total sleep time by 30 minutes or more and they have occurred in the last 3 days, I say the person has experienced Acute Sleep Disruption.
Poor sleep environments (e.g., too noisy, too warm, too cold, too light), stimulants like caffeine and nicotine or sedating substances like alcohol and medication can all result in significant disruptions to your sleep quality. These disruptions can be hard to detect because they don’t shorten the quantity of sleep you get. They change the amount of deep sleep and REM sleep you get. To most people all sleep feels the same, so it feels like your sleep has not been disturbed. If, during the last 3 nights, your deep sleep (stages 3 & 4) dropped below 10% per night or your REM sleep dropped below 15% per night then you have experienced Acute Sleep Disruption, even if you still got the right amount of sleep. When you get less deep sleep or REM sleep, you get more light sleep (stages 1 & 2). Light sleep does not provide the same restorative benefits. Without these benefits, you can become fatigued during the day and your performance may suffer.
Reference
[1] See for example: Anch, A., Browman, C., Mitler, M., & Walsh, J. (1988). Sleep: A Scientific Perspective. New Jersey: Prentice-Hall.