A cabin can add more to the cost of a crossing than the crossing itself. It can also be the difference between arriving in northern Spain ready to drive and arriving feeling as though you've spent the night in an airport departure lounge. Both things are true, which is why the honest answer comes down to four questions: how long you're at sea, when you sail, how many of you there are, and what you have to do the next morning.

What the extra money actually buys

On most long routes, the choice isn't cabin or nothing. It's cabin or a reclining seat in a shared lounge, and it's worth knowing exactly what separates them before you look at prices.

Cabins come in grades. The cheapest are usually windowless, with two to four berths and either an en suite shower or a shared washroom along the corridor. Pay more and you get a window, a proper double bed, a television, tea and coffee. On some ships you can buy a single berth in a shared cabin rather than the whole room, which often undercuts everything else — though you'll be sharing with strangers.

The detail that decides most arguments is this: cabins are priced per cabin, not per person. A four-berth cabin split between four people can cost each of you less than a decent meal ashore. The same cabin booked by one person costs four times as much per head.

Privacy, sleep and the small hours

Reclining seats are fine for a few hours. Overnight, the lounge becomes a different place. Lights stay dim rather than off. Doors bang. Announcements come over the speakers. Somebody's phone alarm goes off at four in the morning for a crossing that docks at seven. You can lean back perhaps a third of the way, which means sleeping sitting up with your bag trapped under your knees.

A cabin gives you a door that locks, a switch you control, and a bed you can lie flat in. On an eight-hour crossing that is the difference between five or six hours of genuine sleep and an hour or two of broken dozing. If you're prone to seasickness, a lower bunk amidships moves far less than a chair near the front of the ship, and you have somewhere private to sit it out.

The smaller wins matter too. You can shower before the drive. You can leave the children asleep while you get dressed. You can shut the door on the party in the bar.

The honest case against paying for one

It isn't always worth it, and brochure photographs of crisp white duvets don't help you judge.

Short overnight hops are the clearest example. If you board at eleven and disembark at six, you're paying for a window you spend queuing, finding your cabin, unpacking, sleeping and then queuing again. Two of those hours vanish before you lie down.

Daytime sailings rarely justify the cost, unless you're crossing Biscay in winter and want somewhere to retreat to. Rough weather can defeat the best cabin. And some cabins are genuinely cramped: four berths in a space the size of a walk-in wardrobe, with beds too short for anyone over about six foot. Check the berth dimensions before booking, not after.

Doing the sums properly

Divide the cabin supplement by the number of people travelling, then by the hours you'll realistically be asleep. Then ask what that money would otherwise buy.

If the supplement is roughly the cost of a night in a budget hotel, and the alternative is arriving at seven in the morning with a five-hour drive ahead, the cabin is usually the better purchase. If it's the price of a full day of your holiday and you're only on board for six hours, it usually isn't.

Weigh the arrival day honestly. A night in a recliner is survivable. Tiredness behind the wheel is not a small matter, and if the first thing you'll do after disembarking is drive a long way, factor that in. If you suffer from motion sickness, ask a pharmacist for advice before you travel — remedies work best taken well before you sail.

Where a cabin usually earns its keep

Some crossings are built for cabins and others simply aren't. As a rule of thumb, the cabin starts to pay for itself once you have six or more hours on board overnight.

• Overnight North Sea crossings, where the ship is your hotel for the night and the alternative is paying for one ashore anyway.
• Long Irish Sea sailings, particularly if you're continuing by car the next morning.
• Western Channel and Biscay routes to France and Spain, where the voyage covers a full night and part of a day.
• Any sailing that docks before dawn, especially with children or a long onward journey.
• Trips where the following day matters — a wedding, a meeting, a flight connection.

Family cabins, adjoining cabins and accessible cabins exist on most large ships, but they're limited and sell out first. If you need one, book as soon as your dates are fixed.

When to keep your money

Skip the cabin on crossings of five or six hours or less. Skip it on daytime sailings. Skip it if you're travelling alone on a tight budget and can sleep sitting up —

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