I was on dexedrine for years and insomnia was a huge problem for me, so I'm share all my experience and knowledge here, and I'm hoping everyone will pitch in so this will turn into a goldmine of information, something that is really needed on the internet. If it gets big enough, we can turn it into a proper information source which no doubt be the most extensive sleep aid information source to be found on the internet.
Alcohol:
I'll start with the most widely available sedative that everyones familiar with. I found this to be a terrible solution for the amphetamine comedown and insomnia because I would need a lot of it to put me asleep, so I'd have to choose between drinking loads of beer which would fill my bladder (resulting in me waking up numerous times throughout the night to take a piss), or stronger drinks like vodka which wreaked havoc on my stomach (I got a hiatal hernia years ago from drinking whiskey straight while on amps). Even with spirits, I still had to drink loads of water to counteract the dehydration and reduce the hangover the next day. Another problem with alcohol is it induces low quality sleep.
Alcohol is not very selective, and has various different mechanisms including GABA_a and GABA_b agonism. A few other small molecule sedatives I'll mention here are chloral hydrate, diethyl ether and chloroform. I hear chloroform used to be commonly used as a hypnotic. A major drawback is it is difficult to dose. I hear it causes headaches and other side effects. I don't know much about diethyl ether, I have never heard of it being used as a sleep aid. Chloral hydrate is supposedly an effective sedative and hypnotic, but tolerance develops extremely fast.
Antihistamines:
The first thing I tried of course were OTC sedatives, so I tried 1st generation antihistamines like diphenhydramine and promethazine, and while I think they helped a bit, they never quite cut it for me. Antihistamines are also vasoconstrictors which doesn't help if you already have high blood pressure. I heard that mixing antihistamines with alcohol is far more sedating, so I tried that during an amphetamine comedown one night and I got a severe case of dyspnea (the sensation of not being able to get enough air, no matter how deeply you breath) which was horrible so I've avoided that combination ever since.
When I'm sober, I find antihistamines to be pretty good hypnotics and pretty sedating. The most sedating ones I'n my experience are doxylamine and chlorphenamine. I've heard of a prescription antihistamine called hydroxyzine which sounds like it is more sedating than the OTC ones. In some countries, scopalamine (an acetylcholine antagonist) is available OTC and I hear it is a decent sleep aid. First generation antihistamines tend to block ACh receptors as well as histamine receptors, so ACh antagonism likely plays a role in their sedative properties.
Opioids:
The first class of drug I found that succsessfully counteracted amphetamine withdrawals and insomnia for me were opiates, the main one I used was codeine and even that worked wonders. It completely reversed the comedown and replaced it with euphoria that I never experience on opiates alone. Taking enough of it would allow me to sleep too. However, it would only help me sleep if I took it well after (over 12 hours) my last dose of amphetamine and I found that it doesn't counteract stimulant effects of d-amp at all. While others claim it to be a great combo, I never combine opioids with amphetamines because I get bad side effects from that combo. Another plus (with respect to counteracting amphetamine side effects) is they dilate the blood vessels, stimulate the appetite, and counteract diarrhea.
Stronger opioids like oxycodone will of course work much better. The downsides to opioids are that they are expensive, hard to obtain and highly addictive. I find that kratom worked well for reversing the comedown, but tended to cause insomnia, rather than alleviate it. When I was not on amphetamines though, kratom was a good hypnotic.
BZ agonists:
BZ agonists include classical benzodiazepines (i.e. valium or xanax) and z drugs (i.e. ambien). The first one I tried was zolpidem (ambien) and I found it to work BRILLIANTLY for counteracting amphetamine comedowns and insomnia. Taking 20mg of zolpidem while coming down from amphetamine is enough to put me asleep for 12 hours straight. And better yet, it doesn't force me to sleep so I can stay awake and still feel energetic if I want to. The problem was that it made me feel nauseous and sick for the whole next day. Besides zolpidem, I tried zopiclone and had the same problem (next day nausea and headaches).
Then I tried benzodiazepines (only ones I've tried are alprazolam, diazepam, etizolam and midazolam) and the effects were perfect. Everything I could possibly want in a sedative. They would completely turn around the comedown, and put me to sleep if I needed to, and the next day I feel fine. Like Z drugs, they don't forcibly tranquilise me, I still have energy while I'm on them so I can continue to work, similar to opioids. Like opioids, BZ agonists are vasodilators and appetite stimulants. In my experience, they combine well with amphetamines and there is synergy (they counteract each others negative side effects).
I believe muscle relaxants like carisoprodol and cyclobenzeprine are also BZ agonists and have sedative and hypnotic properties.
The problem of course with BZ agonists is that they are extremely addictive so daily use rapidly results in dependence, and withdrawals are horrific. Also, they don't induce high quality sleep.
GABA_b agonists:
For a while, I was taking GHB to sleep and it was excellent for knocking me out at night, but it came with a wide range of problems. The dopamine rebound the next day is one of the major problems. Another one is how easy it is to overdose (I overdosed a handful of times during the 3 weeks or so that I used it). It only allowed me to sleep for 4 hours at a time, so I'd have to always have a second dose waiting beside my bed in order to get 8 hours sleep. Then it is extremely addictive, much more so than benzos, although I hear the majority of the withdrawal only last a maximum of a month (unlike benzos which can last many months to even years). However, I hear that one needs to do it 24/7 to get physically dependent, whereas people who only use it at night don't become dependent that easily at all. I also hear that GHB salts are less addicting than GBL.
The other GABA_b agonist I tried was phenibut, which was not as sedating, but still worked well as a hypnotic. Like GHB, I found that it interacted really negatively with amphetamines though, so I would have to wait at least 12 hours after my last dose of amps to take it. The worst thing about phenibut is that its insanely addictive. Just 7 days of daily use is enough to start becoming physically dependent. In my experience it is only good for occasional use, using phenibut regularly is asking for trouble. Its also quite unpredictable and effects everyone differently. Sometimes I found it gave me insomnia rather than helped me sleep.
Misc. GABAergics:
I tried kava and found it to be a good mild anxiolytic/sedative, but wasn't strong enough at all to counteract the insomnia. Other GABAergics I've tried are valerian root, passionflower and picamilon and I didn't find any of them to be sedating at all.
Serotonin antagonists:
Trazodone in my experience is an excellent sedative. Its a forceful sedative in the sense that when it kicks in, I am too drowsy to do anything so I have to just go to bed. It induces a ravenous appetite which is good for people on amphetamines. In my experience, its not as effective at counteracting amphetamine comedown and insomnia as BZ agonists, but it is still one of the most effective sedatives I have ever tried. I found trazodone to have few side effects.
A similar drug is mirtazapine. I have not tried it myself, but like trazodone, it blocks various serotonin receptors and is prescribed as a sleep aid. Its far less selective than trazodone, on top of 5-HT (serotonin) receptors, it also significantly blocks acetylcholine and histamine receptors. That should make it even more effective as a sedative and hypnotic. These drugs are much less physically addictive than benzos, but one still should avoid using them daily for too long.
Antipsychotics:
I have not tried any antipsychotics myself, but I hear they are powerful sedatives and are excellent for bringing one down from amphetamines and inducing sleep. Antipsychotics like quetiapine and halperidol, if I'm not mistaken, work by blocking a wide range of different receptors, including dopamine receptors.
For this reason they come with a wide range of possible side effects including akithisia (which is something I suffer from sometimes myself).
A naturally occurring antipsychotic is reserpine which is an alkaloid found in Indian Snakeroot (Rauvolfia serpentina). Reserpine works by forcing monoamine neurotransmitters (namely dopamine and norepinephrine) out of the synaptic vescicles, into the cytoplasm where they are broken down by MAO. The result is a depletion of these neurotransmitters. Obviously MAOIs would counteract the effects of reserpine.
I realise I'm lumping a wide range of different mechanisms of drugs into this one category. Thats due to my limited knowledge on the subject, feel free to expand on it.
a2 agonists:
a2 agonists like clonidine and tizanidine are prescribed as sleep aids. I have never tried them myself, but I have heard good things about them and want to try them at some point. They work by activating a2 adrenergic receptors and this in turn, inhibits the release of norepinephrine.
Tricyclic antidepressants:
TCAs main mechanism of action for its antidepressant properties is reuptake inhibition for serotonin and norepinephrine, but it is a pretty non selective antidepressant in that sense because it also happens to block various receptors including histamine and acetylcholine receptors. This makes it a good sedative.
Gabapentin/pregabalin:
I tried gabapentin for amphetamine comedowns and insomnia, and it didn't work at all. I don't find gabapentin to be in any way sedating, I find it more stimulating than anything. Other people seem to get sedative effects from it though.
Barbiturates:
Barbiturates bind to the barbiturate site on the GABA_a receptor (as opposed to the BZ site that benzos bind to). I hear they are excellent sedatives but tolerance and dependence develops insanely fast. They are really hard to obtain these days.
Cannabinoids:
In my case, cannabis stimulates me and induces insomnia, but other people claim it helps them sleep, so I suppose cannabinoids act as a sedative and sleep aid for some people. A downside is they make the heart race like mad.
Melotonin:
When I'm sober, I find 10mg of melotonin to be an effective sleep aid, but on amphetamines, it doesn't seem to work. I've heard of novel anxiolytics and antidepressants that work on melotonin receptors. One of them is afobazole, which I believe is an M2 antagonist (among other things). I hear its not sedating at all though. Agomelatine is a melotonin agonist.
Magnesium:
Magnesium blocks NMDA channels in a voltage dependant manner, and therefore prevents glutamate overactivity. Many people claim magnesium supplements help them with sleep, I personally haven't found them to help, although I've only tried magnesium oxide, there are other salts with higher bioavailability.
Miscellaneous:
Here are some miscellaneous sedatives/hypnotics I have heard of but haven't tried. I said miscellaneous because they either have miscellaneous mechanisms of action, or I don't know their mechanism.
1.) Rhoeadine - An alkaloid from the red poppy (Papaver rhoeas) which supposedly has hypnotic properties. Red poppies contain no significant amounts of opioid alkaloids, so I believe they are legal everywhere.
2.) Tetrahydropalmatine - An alkaloid found in various species of corydalis mushrooms. I've seen people selling corydalis extract on ebay, I believe its legal everywhere.
3.) Glaucine - Glaucine is an alkaloid found in several different plant species in the Papaveraceae family such as Glaucium flavum, Glaucium oxylobum and Corydalis yanhusuo, and in other plants like Croton lechleri. I hear it has sedative properties.
4.) Sodium valproate - I've heard of this being used as a sedative and hypnotic but I don't know anything about it.
5.) Validol - I just came across this one now.
6.) Kanna (Sceletium tortuosum) - I hear this is a mild sedative. Don't know its mechanism.
