I think that sleep hygiene is different for auDHDers, but this is anecdotal and based on me extrapolating so don't presume that most of this comment is backed up by hard evidence.

One key factor that gets overlooked is your so-called "sensory diet".

If you tend to be higher masking or you suppress your stims, you may find that you really struggle to centre yourself and to settle enough to get to sleep because your nervous system is dysregulated. I know this definitely affects my sleep onset. It might be worth literally scheduling in some defined stim-time as a wind down exercise for sleep.

With regards to stimming, there's a real gap I've noticed in peer knowledge on the topic. Some forms of stimming are stimulating or "arousing" in the sense of being energising or filling a sensory gap, sorta like when people jump up and shake themselves off to fire up. Then there are stims that are soothing or calming, that help you relax. Things like rocking and humming are often experienced as soothing stims.

When it comes to getting to sleep, I think it's worthwhile identifying which stims you find soothing and focusing on using some of them before bed.

In short, stims come in the following dimensions:

Visual, touch, taste, smell, hearing, your head's orientation, sensations from your body that determine its orientation in space and which feel pressure, and your internal feelings in your body
(Visual, tactile, gustatory, olfactory, auditory, vestibular, proprioceptive, & interoceptive)

Do you find swirling or flowing patterns calming? Maybe make a playlist of things like waterfalls, waves on the beach, this fabulous sand wheel art performance, footage of how ink is made, or if it's within your budget perhaps consider getting a ceiling projector that creates moving illuminated patterns. That sort of thing.

I think touch is pretty easy - find textures that feel soothing on your skin, maybe run a fan in your room if you like the feeling of airflow over your skin, wear comfortable clothing to bed or don't wear anything at all, etc.

I'm very sensory-seeking with taste so there aren't any gustatory sensory inputs that I can think of which are calming but you might be able to for your own experience.

Smell is a really, really good one. It also tends to be extremely specific personally. But your olfactory system links really deeply into your memory so you can leverage this for sleep hygiene and associate a particular smell you find calming with a bedtime routine and you'll find that over time that connection will get very strong and the smell will cue your brain to get ready for sleep very rapidly. My suggestion would be to opt for a particular scent, whether it is artificial or it's an essential oil or a blend, which is replaceable - it's easier to find a different source for the smell of lavender or sandalwood than it is to find a replacement for a particular scented candle which is now suddenly a discontinued line at a retail outlet.

Hearing is a good one too - I'd explore different "colour" noises. White noise is the classic example but a lot of autistic and ADHD folks tends to prefer pink noise or the unfortunately-named brown noise. The sound of rainfall or flowing water can also be really soothing, and there are plenty of noise generator apps available. Crysknife007 on YouTube has a whole lot of different colour noises that are worth exploring too. Deep layered brown noise is a personal fav but you need good headphones or speakers to actually hear it. Fans also produce white noise so that can be soothing too.

With regards to vestibular sensory input, certain types of rocking can be particularly good. Swaying in a hammock, rocking in a rocking chair, and other ways of rocking your body are good options.

With regards to proprioceptive sensory input, the classic option is a weighted blanket. There's also different types of binding or making cocoons that might work for you. Obviously with the more intense examples, remember to be safe with it but sometimes, for example, having a little gentle pressure on your head from a beanie might be what works for you.

Interoceptive sensory input is usually one of the most difficult to connect with for autistic people. Imo eliminating the negative interoceptive sensory experiences are probably the important parts - can you feel if you are hungry, thirsty, sore, tense, emotionally dysregulated, etc? If so, try to address what's going on so that discomfort can be eliminated or reduced. Doing a sort of short interoceptive-focused body scan might be really helpful here.

Last thought on sensory processing for insomnia which is pretty important to auDHDers specifically: there's a minimum level of stimulation that most ADHDers require or otherwise it's borderline physically painful. Yes, a calming environment works well for most sleep hygiene but note that this is specific advice for allistic people. Autistic people, ADHDers, and especially auDHDers tend to require a higher minimum level of (pleasant) sensory input to feel at ease. Do not assume that playing Sigur Ros at bedtime is going to be what's right for you just because it feels like it's supposed to be soothing since it fits our cultural concept of calming because it is slow tempo music which is uncluttered. The heavy-hitting double bass drumming at a rapid tempo from some metal might be what hits the spot for you, and I'm not being hyperbolic in the slightest with this example. Be open-minded and observe how your body responds, do not comfine yourself to the boundaries of cultural norms or expectations.

Moving on, melatonin is good but it's worth understanding how it works from a sleep cycle perspective. Don't take it at midnight, unless your sleep is really out of whack. Try to take it 30-90 mins before your ideal sleep onset time, or even less, depending on if you're trying to shift your sleep onset from late in the night to earlier or if you're content with when you are getting to sleep. Note that some people are just night night owls and perhaps embracing this might be easier than attempting to enforce a typical sleep schedule that is expected by society, especially in the more extreme cases.

Lastly, sleep masks appear to improve quality of sleep based on research. Autistic people are generally very sensitive to light. I feel like sleep masks are probably even more effective for autistic people than neurotypical people because of this fact. No I will not back this claim up with scientific evidence.

(If none of this advice is helpful then as a last resort you could always try reading one of my comments - they go on for fucking eternity, they are drier than Noam Chomsky's throat, and they are boring as hell so maybe this could be a good alternative before reaching for the horse tranquillisers?)