You meet up with your friend or you come home from class. They say hey, you say hey back. You ask, "How are you?" They respond, "Pretty good, how about you?"
You could choose to lie and say that you're doing just fine or you could be honest. You trust this person and perhaps talking about it would help you feel better. You say, "I've been really tired, lately, actually."
"You're always tired, though!"
"You shouldn't be tired; not like you do anything, anyway."
"Maybe you should go to bed earlier, then."
"You're tired? I did [X amount of work] today! What did you do?"
Suddenly, you wished you would have just lied and said you were fine.
There are two different types of being tired.
There is being physically tired. You've been working all day long and haven't had a rest. You stayed up way too late the previous night and only got a few hours sleep.
Then, there is being mentally tired. This tiredness sometimes never goes away. It follows you like an ugly shadow. You could very well sleep for ten hours straight, wake up, and still feel like you could crawl back into bed and sleep for four more. Sometimes you have no idea why you're feeling so low. No amount of caffeine could really stave it off.
Sometimes, you just can't help it. Fatigue is a side-effect of depression and anxiety. Someone responding to you in a way that belittles how you're feeling doesn't help much either.
You may have a friend who you feel is always tired. They take frequent naps, but still sleep at night. They may be able to function throughout the day, but their spark just isn't there anymore. Instead of trying to make their fatigue seem unimportant or just brush it off, try asking them how they're really feeling and see what you could possibly do to help them. They may not realize just how often they're feeling down, or they may not know what to do about it.
Be patient with them. Their fatigue is not going to go away after one night's rest. Be understanding with them. Because of mental illness, their fatigue may never go away, unfortunately. It may just lessen as time goes on.
And if you are the person that stays up half the night watching Netflix and only gets a few hours sleep, please go to bed earlier.