I had surgery when woke up I felt like I only had 20% energy & felt extremely tired. Is this a common sensation? Why do people feel so tired after surgery?
Answers in comments are correct. The tail effect of hypnotic drugs combined to opiates given for pain relief will make you feel really tired just after surgery.
But if you are referring to a more long-term effect, such as feeling tired for several days after surgery, then several factors combine: