Lost sleep can be compensated, but only to a certain extent. If, for example, we loose a night's sleep of 8hrs, we cannot sleep continuously for 16hrs the next day. After a significant sleep loss, we may have more slow-wave sleep (deep sleep) for the next couple of nights, but we rarely sleep more than 2-4hrs longer than usual. Cognitive abilities like paying attention and reacting to a stimulus are reduced even with a moderate reduction of sleep. The person suffers from tiredness and lack of energy, is irritable, grouchy and suffers from excessive daytime sleepiness.