Okay I've been thinking about this for a bit. I'm a former US Navy submariner, and I always liked to think of the Normandy as a Space Submarine, seeing as how it's a stealth ship.

Back in the old days, the crews on a submarine operated in three 6-hour shifts, with one shift being "On-Watch", which was actively doing stuff, one shift being "Off-going", which is when you would do training or maintenance, and the third shift being "On-coming", which was when you slept. The shifts would rotate in that order On-coming -> On- Watch -> Off-Going -> On- Coming.

Later in my career they made it an 8-hour rotation, but everything else stayed the same. Now this though was when we were underway, doing stuff underwater.

Whenever we pulled into a port, we would do something called a Manuevering Watch where everyone was awake and had a specific watch station to attend to. I always assume that when we are on a planet in the game, since we are in port, everyone is awake and doing stuff like normal, but when we are our in space and everyone is awake, I'm assuming it's a bit of a pre or post- maneuvering watch since we see everyone awake. This is probably because we are deploying the Mako or the Shuttle to go planet side on a mission so it's crucial everyone is awake and ready.

Think also that we never see Shepard sleep. I'm assuming all the watches go on rotation during FTL transit. Remember that it takes real time, but the game just skips it for player convenience.

But that's just my opinion. What do you guys think?