In a simple model of the brain and how we manipulate it, stimulants make people more alert, less needful of sleep, shortens sleep inertia, and if given to excess makes it impossible to get to sleep,
While sedatives and hypnotics dull the brain, make us less able to do complex algebra, increase sleepiness, and shorten sleep latency. And make it easier to sleep through the night.
But it's not that simple. The brain and the regulation of wakefullness and sleepiness and vigilance is complex, with many interacting feedback loops. A drug might target one neurotransmitter system, but past a certain dosage or in combination with other drugs and bodily situations, the effect might not be a simple increase or decrease in brain activity.
First, as we know, sleep is not a passive process. It is an active one, and healthy brains sleep better than unhealthy ones. So it should not be a surprise that stimulants can often help people sleep better and depressants cause troubled sleep.
There are also indirect effects. Stimulants can improve mood during the day and help the individual respond to stress. This could translate into better nights. The direct effect of the stimulant might include decreased restlessness, which keeps people from falling asleep.
For example. alcohol is a sedative and certainly makes most people less active, but it also makes it tougher to sleep through the night. Much as sodium oxybate (which promotes deep sleep) is given to narcoleptics who sleep too much, so stimulants may be given to people who sleep too little. By increasing the highs and lows of the arousal response in the brain, sleep can be promoted.
If the flip-flop model is valid and the parts of the brain in the ascending arousal system arising from the hypothalamus interact with sleep-active neurons in the VLPO (ventrolateral preoptic nucleus) , then strengthening both branches could enhance sleep-wake regulation.
Caffeine can help people sleep. You probably know people who claim they can't sleep if they drink coffee past 2 PM or so. You probably meet other people who claim the drink coffee right before bed and have no problems. Is this a contradiction? No. They are probably both telling you the truth.
ADHD people sometimes take prescription stimulants. Ritalin (methylphenidate) and Adderall (a mix of amphetamines) and nowadays Nuvigil (modafinil). More than half of children with ADHD have sleep problems, especially insomnia.
Anecdotal reports indicate many ADHD people start sleeping better after starting the stimulant regimen. Formal experiments confirm that this is true:
German scientists tested attention-deficit/hyperactivity disorder patients with polysomnograms. When the patients were given methylphenidate their sleep qualty improved. Their sleep latency and efficiency improved, and in interviews the patients said they felt more rested.
The group of patients who took the methylphenidate had substantially lower average time to fall asleep. They also spent less time in REM, which is a similar finding to when patients are given antidepressant drugs.
Paradoxical effects are common if you pay attention to pharmacology literature. The prevalence of ADHD in adults is 4 to 5%. The 95% of the population that does not have ADHD can have varying responses to stimulants.
As mentioned, caffeine can work both ways. Other stimulants probably also can work both ways, although the effect may vary depending on the individual's experience with the drug. Even when caffeine works against sleep, it works better during the day than at night.
Oftentimes, a first dose of Adderall or Nuvigil will cause insomnia. A more experienced user may find the substances aid them in getting to sleep.
The dose, timing, and other physiological conditions play a part in whether the stimulant will result in insomnia. In general, it is a fallacy to assume that a higher dose of a drug results in an amplification of the effects of the same drug at a low dose.
"Stimulants" and "sedatives" are reductive categories. It is incorrect to think of these drugs as on a spectrum of causing excitement/sleepiness.
The federal government's website warning about abuse of stimulants names insomnia and disturned sleep as effects of these drugs.
We are unaware of studies that show nicotine can improve sleep, although in addicted smokers, withdrawal can certainly cause insomnia.