Hypnopompic hallucinations, in which an individual perceives visual or aural illusions upon waking, are generally attributed to impaired function in the frontal lobe. The phenomenon, often referred to as a manifestation of "sleep inertia," occurs when the brain is unable to quickly readjust an individual's dreaming state to an awake one. As a result, images and sounds from dreams can be carried over into a person's conscious perception. Insomnia has been identified as the primary cause for the impaired mental function associated with these types of hallucinations, although other possible causes include drug abuse, depression, and physiological disorders.
Individuals suffering from insomnia are the most likely to experience hypnopompic hallucinations. Sleep deprivation can severely slow mental function down, resulting in the frontal lobe's inability to quickly distinguish vivid dreams from reality. In addition, insomniacs often suffer from microsleeps, extremely brief periods of unintended sleep. The rapid rate of sleeping can immediately plunge an individual into rapid eye movement (REM) sleep, the level of sleep most associated with dreaming. A microsleeping individual usually wakes several seconds after reaching the state, accelerating through the stages of sleep and consciousness too quickly for the brain to process.
The use of certain drugs can also contribute to slowing down brain function. This is most apparent with sedatives, such as diazepam. Those under heavy sedative medication usually report sluggish mental function, which, in turn, can lead to hallucinations upon waking from sleep. Hallucinogenic drugs like opium, on the other hand, contribute to the development of these illusions by impairing the user's ability to distinguish fantasy from reality. It is possible that this effect can carry over after the drug's effects have worn off.
Mental disorders that involve the shutting down of certain mental processes can have hallucinogenic effects after waking. Depressive disorders, such as major depressive disorder (MDD) and seasonal affective disorder (SAD), are possible causes of hypnopompic hallucinations. These conditions are often associated with bouts of insomnia and a loss of mental acuity, two major contributing factors to sleep-related hallucinations.
In the case of MDD, there is a large possibility that hallucinations might result from biological problems in the brain. This disorder can be the result of a physiological inability to produce or maintain adequate levels of serotonin, for instance. This leads to impaired frontal lobe activity and the hallucinations as a result. Some individuals might have weakened frontal lobe function as a result of physical damage or congenital disorders, making them more likely to experience hypnopompic hallucinations when waking.