I’ve done this trip a few times now, and there are definitely ways to make it smoother. First, if you haven’t already, book a cabin rather than a reserved seat. It’s worth the extra cost to have a private space where you can lie down properly and get some decent sleep. Bring a few comforts from home, like your own pillow, especially if you’re a picky sleeper. Snacks and water are also handy to have. Although the ferries usually have dining options, it’s not always open at odd hours, so having something on hand helps. The onboard restaurants are usually decent, with options ranging from casual snacks to full meals.
For more specific details on the overnight ferry routes, dining options, and general tips for first-timers, I found https://ferrytospain.co.uk/ to be really useful. They cover everything from cabin types to what’s available in terms of food and entertainment, so you can get an idea of what to expect. Knowing all this made my last trip a lot less stressful!