Wakefulness

Wakefulness is a daily recurring state of consciousness. In this state, an individual is conscious of their surroundings. They can engage in coherent cognitive and behavioral responses to the external world. This state is the functional opposite of sleep. During sleep, the brain excludes most external inputs from its neural processing.

Being awake causes specific changes in how brain cells function. These cells are called neurons. As the brain stays awake longer, the firing rates of cerebral cortex neurons increase. These neurons also exhibit greater synchronous firing. This means they fire together in a more coordinated way. When a person has had a sustained period of sleep, these rates change. Both the speed and the synchronicity of the neuron firing decrease.

Wakefulness also affects how the brain manages its energy. Neurons require a constant supply of power to function. This energy is supplied by specialized cells called astrocytes. These astrocytes hold a stored energy source known as glycogen. Being awake leads to a reduction of the glycogen held in these astrocytes. Research suggests that one of the underlying functions of sleep is to replenish this glycogen. Sleep allows the astrocytes to refill the energy needed for the neurons.

The maintenance of wakefulness requires a complex interaction of systems. These systems consist of multiple neurotransmitter systems. The process begins in the brainstem. The signals then ascend through the midbrain, the hypothalamus, the thalamus, and the basal forebrain. The posterior hypothalamus plays a key role in this process. It is responsible for maintaining the cortical activation that underlies wakefulness. This specific area helps control the shifts between being awake and being asleep.

Scientists have identified specific neurons that drive these changes. One major system uses histamine within the tuberomammillary nucleus. These histamine neurons also exist in the adjacent posterior hypothalamus. They project signals to the entire brain. This is currently the most wake-selective system identified in the brain. Another key system involves orexin-projecting neurons. Orexins are also known as hypocretins. These neurons exist in areas adjacent to the histamine neurons. Like histamine neurons, they project widely to most brain areas and associate with arousal.

Research suggests that histamine and orexin play complementary roles. They are distinct but work together to control wakefulness. Histamine is primarily involved with cognition and the activation of cortical EEG. Orexin is more involved with regulating wakeful behavior. A deficiency in orexin has a direct medical consequence. It has been identified as the cause of narcolepsy. This shows how vital these specific chemical systems are for maintaining a steady state of consciousness.

The timing of when wakefulness begins is also a subject of study. Some research suggests that a fetus is not awake. Wakefulness may actually occur in the newborn. This transition might be triggered by the stress of being born. This event is associated with the activation of the locus coeruleus. This suggests that the very beginning of our conscious life is tied to specific biological triggers during birth.