Ear plugs - not just noise cancelling headphones. It's much easier to sleep with ear plugs than with headphones. And headphones can get uncomfortable after many hours. And if you want extra quietness you can use both the earplugs and headphones at the same time
On my list to try but haven't yet: wax earplugs. They are single-use, so more costly over time. But I have been told they don't build up pressure like foam or silicone plugs do.