Cognitive Performance And Sleep

Four major sleep related-factors affect our cognitive performance

i) homeostatic drive for sleep at night (this is determined above the number of consecutive hours that we've been awake. The longer we are awake, the stronger is the drive for sleep. If the homeostatic pressure to sleep becomes high enough, the brain "switches off")

ii) total amount of sleep-deficit accumulated over several days (an accumulating sleep deficit makes it more difficult for the brain to function. Furthermore, the brain needs to wind down before sleep starts; the impact of sleep-deficit is similar in impact to drinking too much alcohol)

iii) our body clock (circadian and ultradian rhythms) determines our energy and concentration levels during the day.

- Circadian rhythms control whether we get a good, restful sleep and are dependent upon many factors, such as sunlight on our eyes, body temperature and regular sleeping patterns.

- Ultradian rhythms last for 90 minutes, send us in and out of deep sleep during the night, and are linked to our alertness during the day. Usually, after 90 minutes, performance dips and the ability to think cognitively decreases. Furthermore, performance is linked with glucose levels which are dependent on what, and when, you have eaten.

It is of interest to note that the homeostatic drive for sleep works in the opposite direction to the circadian pacemaker:

"...the circadian pacemaker sends out its strongest drive for sleep just before we habitually wake-up, and its strongest drive for waking one to three hours before we usually go to bed, just when the homeostatic drive for sleep is peaking..."

Charles Czeisler, 2006

Reasons for this are not clear.

"...In the midafternoon, when we are already built a substantial homeostatic sleep drive, the circadian system has not yet come to the rescue......the circadian pacemaker sends out a stronger and stronger drive for waking as the day progresses. Provided you're keeping a regular schedule, the rise in sleep facilitating hormone melatonin will then quiet the circadian pacemaker one to two hours before your habitual bedtime, enabling the homeostatic sleep drive to take over and allow you to get to sleep. As the homeostatic drive dissipates midway through the sleep episode, the circadian drive to sleep increases toward morning, maintaining our ability to obtain a full night of sleep. After a usual wake time, the levels of melatonin begin to decline. Normally, the two mutually opposing processes work well together, sustaining alertness throughout the day and promoting a solid night of sleep..."

Charles Czeisler, 2006

iv) sleep inertia (the grogginess most people experience when we first wake-up), ie

"...the part of your brain responsible for memory consolidation doesn't function well for five to 20 minutes after you wake-up and doesn't reach its peak efficiency for a couple of hours.... There is a transitional period between the time you wake-up and the time your brain becomes fully functional..."

Charles Czeisler, 2006