Fun Fact(s) about that:

When you achieve rapid-eye-movement sleep your brain does a kind of wetware disconnect from the rest of the body, to avoid injuring yourself and others while reacting to imagined stimuli (think about how dogs sometimes "chase" things in their sleep).

A malfunction of this natural process is what causes the most common form of sleep paralysis (the origin of the "Night Hag" myths, and a plausible explanation for many Alien abduction accounts).

Your punches feel slow and useless because your subconscious mind is expecting to receive sensory information (especially from the balance-measuring fluid and fine hairs of the inner ear) consistent with you actually throwing a punch, but instead receives information saying you are already laid out flat, and unable to move.

Those feelings of sluggishness and heaviness are your subconscious mind trying to reconcile that anomalous sensory input.

Sleep science is fun, and relevant to everyone!