A quick, honest answer first: there's no general UK law that bans riding a legal green lane after dark. A BOAT is a public highway, and public highways don't come with a curfew. That said, "legally allowed" and "actually a good idea" aren't the same question, and this one's worth taking seriously on both counts.
The legal position
The same requirements that apply during the day apply at night, a road-legal, taxed, insured bike, the right licence, and working lights, which matters more here than it ever does in daylight. Nothing about the byway's legal status changes once the sun goes down. The one thing worth actually checking, rather than assuming a blanket answer covers every lane, is whether the specific lane you're planning to ride carries its own Traffic Regulation Order with a time-based condition attached. These are uncommon but not unheard of, and it's exactly the kind of detail a lane's own current status can carry that a general answer like this one can't. Holloway's Legal Status Search on any lane's detail card is built for precisely this, a quick way to check a specific lane rather than relying on a rule of thumb.
Why it's still worth thinking twice
The practical case against night riding is stronger than the legal one. Hazards that are obvious in daylight, a rut, a submerged rock, a low branch, are genuinely harder to spot and judge by headlight alone, and a green lane's whole appeal, the unpredictable surface and the lack of any barrier or kerb to warn you, is exactly what makes reduced visibility riskier here than it is on a lit road. If something does go wrong, you're also further from help and harder to find than you would be on tarmac.
There's a second, less obvious reason worth weighing too: noise carries much further at night, and a bike working hard on a technical section after dark is a far more noticeable, more likely-to-be-reported disturbance than the same bike at 2pm on a Saturday. Noise complaints are a genuine, real contributor to lanes being restricted in the first place, we've covered the mechanics of that properly in our lane etiquette guide, and riding late is one of the more avoidable ways to trigger exactly that outcome.
If you are going to ride after dark
Check the specific lane's current status rather than assuming, make sure your lighting is genuinely adequate rather than just legal, ride well within what you could actually see and react to, and be more conservative than usual about noise near anywhere people live. None of this is about being told not to, it's the same judgement call as riding in poor weather, legal, but worth a more honest assessment of the actual conditions than "the rules allow it" alone gives you.
Check the current status of a lane near you, browsing and checking legal status is free, no account needed.