Vigilance is a term for long-term relaxed alertness and is becoming a buzzword for a desired trait among the human performance community. The military, for instance, wants to find ways to make soldiers vigilant so they can perform at a high level when needed, spontaneously. Rest and sleep are needed to be vigilant.
Sleep researchers and enthusiasts are interested in how sleep quality and duration affect vigilance. Excessive daytime sleepiness, the hallmark of so many sleep disorders, is obviously detrimental to vigilance. So are microsleeps.
If sleep is a mystery which scientists don’t fully understand, the functions and performance of the waking brain are not a whole lot clearer. Behavioral tests are useful in judging how vigilant a person is at a given time. Predicting vigilance and crafting regimens to promote vigilance are harder.
Nobody doubts vigilance is a product of many interacting systems in the nervous system and the rest of the body.
One possible working definition of vigilance relates to how much energy is latent in the waking brain and available to direct concentration. Tonic alertness is another description.
"Attention:, "arousal", "alertness", and vigilance are terms we use from a behavioral view to describe human mental stances. These do not map directly to brain states scientist can identify through imaging techniques. They overlap, too, and there is no firm consensus about what those terms mean. Nevertheless, vigilance is a term professionals use to describe the ability to focus attention over a period of time. How the brain focuses attention is still a mystery, although neuroscientists are making progress at understanding it at a microscopic level.
Stimulants can make people more vigilant. Amphetamines and methylphenidate have been used for decades by the military, students, athletes, and others looking for more vigilance. The drug company Cephalon first sold modafinil in the United States under the name Provigil or Nuvigil – a name derived from promote vigilance. Caffeine is the great vigilance drug in modern life – the cup of coffee pick-me-up.
It is not so simple as longer-sleep=greater vigilance. Animals who are herbivores tend to sleep less than carnivores and are more vigilant during the day – they have to be to avoid predators. And people who sleep long have no necessary vigilance advantage over shorter sleepers. The relationship between sleep time and vigilance is one of the mysteries of sleep.
EEG can be used to monitor the waking brain, as with the sleeping one. Vigilance investigators look at EEG although it is nor accurate to say vigilance is just a condition of high activity in the cerebral cortex. Other physiological indicators scientists look at to get a bead on vigilance are eye movement and autonomic nervous system activity.
A Dutch researcher found that an increase in the theta power of EEG in the waking brain predicts a decrease in vigilance. http://repository.ubn.ru.nl/handle/2066/19548 An increase in beta2 EEG power happened after mental effort and is normally present in patients with narcolepsy, suggesting they have to exert more effort just to do normal daily functions.
The researcher Annika Smit broke subjective alertness and fatigue into four dimensions:
Energy plays a part here - energy as it subjectively feels to the person and to external observers. Sustained attention – use of vigilance – seems to deplete potential and vigilance declines. This is what it means to be mentally tired.
High levels of activity in the brain’s cortex as measured by EEG could be called "arousal" according to Smit, but vigilance as properly understood needs more – including systems in the brain primed for cognitive processing.