When you
wake up after a good night of
sleep, and you feel
groggy and incapable of performing even the simplest tasks for a few
moments, that is sleep inertia. Normally, sleep inertia isn't a
problem, we get over it and
drag ourselves out of bed. However, in the
military, sleep inertia can be a problem if there's an
emergency and troops need to be ready
immediately.
Most people, when woken up suddenly by such a situation, won't experience much sleep inertia at all, since adrenaline quickly kicks in. However, those who are affected by sleep inertia despite that can lag a military unit behind. This can be quite a problem.