There are two shifts offshore: Dayshift or Nightshift.
These usually run from 0700 to 1900 hrs for platforms (plus flotels moored to them) and 1200 to 2359 hrs on Vessels.

The pro's and cons of being on dayshift over nightshift and vice versa are legion.
Dayshift dreamers will have great pleasure in telling of how they will have all the night-time telly to watch when they come off shift. The weather tends to be warmer and usually better while the greater workforce numbers in day mean plenty of hands to assist with something if you have problems.

The Nightshift Owls will often argue that the increased pay (nightshift tend to be paid more) is a big attraction. Less people on nightshift mean it's quieter and more peaceful. It also means that there are less supervisors peering over your shoulder, you often get left to get on with the job unsupervised. In the tropics at night the night-air is a break from the heat of the day. For the flight home night-shift are usually on the first helicopter of the day.

Ideally you'd have your own cabin offshore. This is quite rare offshore unless you're on a modern platform, the reality is you'll be sharing one with another person (In some cases more than one).
If you're both working a dayshift you'll be sharing the cabin in close-proximity to one-another.

To make things easier though a good installation or vessel will have the cabin occupants on a split shift. One on dayshift and the other on nightshift. This means you'll have the cabin to yourself during your 'off-time' of 1900 to 0700 hrs if you're day-shift. The down-side to this is that during your break periods 'in-shift' you'll be forbidden from entering your cabin, or else risk waking up you cabin-mate.

Some people take to night-shift well and thrive on it, others dislike it intensely and struggle to get their body-clock and sleep-patterns to adjust. It can be a relief when a shift-change or 'swing-shift' is given out to a worker for manning reasons. When this happens you go from day to nights or vice versa. The accepted method for changing over is to 'turn-two' which means stop work at 2pm for changing over to nightshift (or 2am for day-shift changeover) to rest and sleep if possible until nightshift begins at 7pm. Make no mistake, you'll be feeling utterly knackered at the end of your 'swing-shift' but will sleep like a rock once your shift ends! For under-going this grueler of a shift-change you should be paid 8 hours or so extra money.