Somotogravic illusion happens. It can happen in a piston twin, it can happen in a 737. If you are well rested you can process the information being presented to you by the instruments effectively which helps you to recognise that you need to maintain the correct attitude on the pfd. If you are processing visual information slowly due to fatigue or distraction you are less likely to make appropriate control inputs in a timely fashion. When you do make inputs they may be more course than is desireable.