Final answer:
CNS stimulants are given 4-6 hours before bedtime to counteract sleepiness and promote wakefulness by increasing neural activity in the central nervous system.
Step-by-step explanation:
CNS stimulants are given 4-6 hours before bedtime because they promote increased levels of neural activity in the central nervous system. These stimulants, such as amphetamines, can help individuals with sleep disorders like narcolepsy, which is associated with reduced levels of the neurotransmitter hypocretin. By taking the stimulants before bedtime, it helps to counteract sleepiness and promote wakefulness.