Arousal is an indication of mind and bodily excitement and receptivity to inputs and stimuli. The aroused organism is physically stronger, quicker, and more able to deal with the world than the unaroused (or sedated) organism. Human mood can be good (exuberant) or bad (angry) when we are aroused.
In the context of sleep, we more or less equate wakefulness with arousal.
We know instinctively that during the day a person’s degree of arousal varies. Most people have a mid-afternoon lag, most people are wide awake in mid-morning and early evening, and different people are different chronotypes so their wakefulness may have ups and downs at different times of day. Environmental factors and interactions with other people affect alertness as does context in many ways. After a big meal we might be less alert. When engaged in a challenging game or situation at work, we might be more alert. While circadian cycles play some part in alertness level, a person’s life circumstances matters as much or more. Does intelligence vary during the day? Yes, any measurement of intelligence and emotions varies over time. And how a person has been sleeping recently, how long he or she has been awake, and the position in the circadian cycle play a part.
Too much arousal may be associated with stress which can have long-term health repercussions. Stress may be a problem mostly because the stress persists; middle-aged and older bodies have a hard time shifting down from stress. Hyperarousal is a major reason for many cases of insomnia. The body and brain cannot downshift from daytime arousal when the Sun goes down. Indeed, one strategy for fighting insomnia is countering orexin neurotransmitters that keep us alert. The neurostransmitters acetylcholine, dopamine, glutamate, histamine, norepinephrine, orexin, and serotonin all promote alertness, and there are probably other substance not yet discovered or appreciated that are important in the arousal system.
Vigilance adds a degree of cognitive ability to this sense of arousal. Factors that influence vigilance include how acute a person’s senses are for taking in external stimuli, brain processing speed, motivation, distraction by other stimuli, social context, and physical environment and comfort. Vigilance implies a degree of steadiness; people who have taken a stimulant like cocaine are alert, but not vigilant.
The most common physiological measure of arousal is the EEG, but there is no physical indication of vigilance and it must be measured through behavioral tests.
Let’s define tonic and phasic in this context. Tonic refers to a state that is stable or only slowly changing over time. Phasic refers to more short-term reaction to changes. Vigilance is a tonic state; a person is vigilant even when there is no stimuli. A vigilant person is on the lookout for stimuli but in a resting state.
When a person is sleep deprived, his or her arousal is lower and so is vigilance.
Brain science researchers most often use the term vigilance to describe an ability to sustain attention to a task for a period of time. Another use refers more to skittishness that an animal may exhibit when in fear of predators.
“Hypervigilance” to describe PTSD victims is sort of like this. Brain scans show activation of the cerebral cortex – more energy use – when the person is aroused.
Related words are attention and alertness. Attention usually refers to a more focused activation of cerebral cortex that enhances information processing. Alertness is another term that overlaps with arousal but more specifically includes some cognitive processing.
A person can be more or less vigilant at the same degree or arousal. But it is safe to generalize that a decline in arousal results in a decline in vigilance. If you disrupt the ascending arousal system, you get lower cognitive performance and a tendency toward sleepiness.
Anatomists have identified the brain stem, basal forebrain and hypothalamus as containing systems that lead to arousal. Sleep scientists point to the ventrolateral preoptic area of the hypothalamus (VLPO) as a key area associated with waking up. The higher thinking functions are associated with the cerebellum and cerebral cortex. These higher functions must be engaged for a person to be vigilant.
So we can think of the vigilance as the thinking man’s arousal. The sophisticated arousal. The gentleman’s arousal.
The orexin system is important in arousal. It also seems to be activated (becomes stronger) by sleep deprivation. This increase in orexin levels may allow the animal to get by in period of short sleep, such as emergencies. Some scientists believe – or at least its activation during sleep deprivation – is a recent evolutionary development which led to an animal such as a human to maintain wakefulness for 16 hours before getting sleepy. Other animals have orexin neurotransmitters in their brains, but few normally stay awake as long as humans do.
The flip-flop switch does a good job or making us either awake or asleep. We awaken rapidly, but cognitive ability upon awakening is low; we are not vigilant. Vigilance rises and falls through the day.