On-duty napping is called controlled rest because there is a set procedure that controls what is done before, during and after the nap. The procedures are designed to increase the likelihood that the nap will fight fatigue and not put the napper at an increased risk of causing an accident due to sleep inertia’s impairments like poor memory, reaction time, speed of information processing and logical reasoning.
If you are developing a controlled rest procedure, here are 3 rules to keep in mind.
Rule 1: Some sleep is better than no sleep. As little as 10 or 15 minutes of sleep can act as a fatigue countermeasure. It won’t buy you a lot of operational time, but it will fight fatigue. What if you are the type of person who does not nap because you feel that you never fall asleep? It is still a good idea to try to nap. If you don’t feel that you fall asleep during your naps, but you do feel relaxed during your naps, there is a good chance you are actually sleeping. Stage One sleep does not feel like sleep, but is still offers fatigue fighting powers. So as long as you feel relaxed, a nap might be doing you some good.
Rule 2: The second rule is that more sleep is always better. Think about how you feel when you have to cut your sleep short to wake up for an early meeting, or how you feel when you stay up too late watching a movie but still have to get up at the same time the next day. You feel better when you get your needed amount of sleep and you feel less fatigued. Napping is no different, longer naps are better at reducing fatigue. There is also a mathematical argument for longer naps. If humans need about 8 hours of sleep for every 24 hours of life, that means 8 hours of sleep buys you 16 hours of safe, healthy and productive wakefulness. In other words, one hour of sleep buys you 2 hours of wakefulness. This means that a longer controlled rest period will be more effective than a shorter one at fighting fatigue and providing a longer post-nap operational period.
Rule 3: The last rule is that sleep inertia needs to dissipate before a napper can be re-engaged in safety critical activities. If this does not happen, it can lead to disasters. In almost all conditions, and especially if the napper uses strategies like physical movement to overcome sleep inertia, the deleterious effects fully dissipate within 20 minutes. This means that, although you do need to manage the risk of sleep inertia, you don’t have to restrict controlled rest to 40 minutes or less as indicated in many guidelines.
The idea behind limiting a nap to 40 minutes is that, within this time frame you will experience some Stage One and some Stage Two sleep, but no Stage Three sleep. This means you won’t be waking up from deep Stage Three sleep and the effects of sleep inertia won’t be quite that bad. The problem is that knowing exactly when a person will enter deep sleep is impossible. It depends on factors like how long they have been awake, how much sleep they are running on and their circadian alignment. In fact, if you look at the average night time sleeper, they enter deep sleep within 30 to 45 minutes. This means that 40 minutes of controlled may increase the severity of sleep inertia. Other research shows that a 50 minute nap results in worse sleep inertia than 20 or 80 minutes naps. Rather than trying to find just the right nap length, it is better to use Rule 2 to guide nap length and get as much sleep as operationally feasible and then factor in a 20 minute sleep inertia dissipation period.
 Folkard, S., Knauth, P., Monk, T., & Rutenfranz, J. (1976). The effect of memory load on the circadian variation in performance efficiency under a rapidly rotating shift system. Ergonomics, 19, 479-488.
 See for example: Tassi, P & Muzet, A. (2000). Sleep inertia. Sleep Medicine Reviews, 4(4), 341-353.
 Evans, F. & Orne, M. (1976). Recovery from fatigue. Annual Summary Report No. 60. Fort Derrick, MD: US Army Medical Research and Development Command.
 See for examples:
(A) Naitoh, P. (1981). Circadian cycles and restorative power of naps. In L. Johnson, D. Tepas, W. Colquhoun, & M. Colligan, (Eds.), Biological Rhythms, Sleep and Shift Work. New York: Spectrum Publications.
(B) Stones, M. (1977). Memory performance after arousal from different sleep stages. British Journal of Psychology, 68, 177-181.
 For a review see: Lovato, N., & Lack, L. (2010). The effects of napping on cognitive function. Progress in Brain Research, 185, 155-156.
 See for examples:
(A) A11F0012, Pitch Excursion, Air Canada, Boeing 767–333, North Atlantic Ocean, 14 January 2011. Transportation Safety Board of Canada.
(B) Report on Accident to Air India Express Boeing 737-800, Mangalore, 22 May 2010. Government of India Ministry of Civil Aviation.
 Balkin, T., & Badia, P. (1988). Relationship between sleep inertia and sleepiness: Cumulative effects of four nights of sleep disruption/restriction on performance following abrupt nocturnal awakenings. Biological Psychology, 27, 245-258.
 Stampi, C., Mullington, J., Rivers, M., Campos, J., & Broughton, R. (1990). Ultrashort sleep schedules: Sleep architecture and the recuperative value of multiple 80- 50- and 20– min naps. In J. Horne (Ed.) Sleep, Bochum, U.K.: Pontenagel Press.