I circled the North Atlantic last Aug/Sept: HAM-Orkney-Faroe-KEF-Greenland-Canada-Ireland-HAM. Yes, there were nights at sea with pitching and rolling, creaking and groaning
I prayed would pass quickly, but never did I feel in any danger.
The longest (and easiest) stretch at sea was 3-4 days, between Canada and Ireland. The most "turbulent" were the 1-2 nights in the north, north seas.
Would I do it all over again? In a heartbeat! Like everyone else said, be sure to take meds. BTW, pills were available at the concierge's desk on board - two types: one "strong" and one "less strong". I opted for the less strong which worked like magic and... turned out to be ginger capsules!
How many nights at sea between EU and Brazil?