Final answer:
The sleep disorder linked to a lack of the neurotransmitter orexin, which is associated with alertness, is narcolepsy. This disorder is characterized by uncontrollable sleepiness during waking hours and can include sudden loss of muscle tone, sleep paralysis, and vivid hallucinations.
Step-by-step explanation:
The neurotransmitter orexin is heavily linked to the regulation of sleep and wakefulness, particularly in terms of maintaining alertness. A deficiency in orexin has been closely associated with the sleep disorder narcolepsy. In patients with narcolepsy, they experience an uncontrollable urge to sleep during normal waking hours, along with other symptoms such as cataplexy (a sudden loss of muscle tone), sleep paralysis, and vivid hallucinations. These symptoms mirror aspects of rapid eye movement (REM) sleep, which is the sleep phase where dreaming usually occurs.
Unlike narcolepsy, other sleep disorders such as sleep apnea, sleepwalking, insomnia, and paradoxical sleep (which is typically used to describe REM sleep where the brain is active but the body is immobile) are not directly linked to a lack of orexin. Sleep apnea, for instance, is characterized by pauses in breathing due to physical blockages during sleep, and does not typically involve the neurotransmitter orexin.