Don't fly the bus but my guess is a turbulence mode is engaged. An auto flight system reacts to altitude excursions at a particular rate. The corrections can be somewhat abrupt especially in short coupled, rapid wave activity. This can be uncomfortable to the pax, and could inhibit the ability to sleep in the case you observed. With the turb mode on, in general terms the AP reacts a bit slower to return to assigned altitude, making the ride/corrections a bit smoother but taking a bit more altitude to do so.
FWIW, the automation may at lower altitudes deselect the turb mode itself, or the pilots hopefully will deselect it when not needed or when approaching the terminal environment. The mode (on some acft) will also reduce the rate of turn if selected on. In the terminal area (where it should be off) If you notice an aircraft turning to an assigned heading slowly, or starting a turn to intercept a course earlier than what you might expect, ask them if the turb mode is on. The answer of course will be no, followed immediately by an increased rate of turn after they turn it off.