I'd really like to take the word JUST out of the lexicon. I don't know if people realize how insensitive they are being. The word JUST belies everything the bipolar person is going through. Further, the word JUST implies that the bipolar is malingering and that there's nothing really wrong that a nice nap couldn't cure.
BOLLOCKS!!! as our friends across the pond would say.
Imagine having a great night's sleep. You awaken. You have no desire to sleep further. Your mind is churning, the more so because you're bipolar. But every fiber of your being resists movement. Your body feels as though it is composed of wet sand. Every movement requires monumental effort. It isn't pain. It isn't muscle or joint stiffness. It is fatigue of overwhelming proportions. This, in turn, brings on lethargy and apathy which will feed the depressive side of the bipolar. All you want to do is curl up in bed, not necessarily to sleep, but to find an off switch, somewhere.
No, it's not "all in my mind" any more than any other ailment. Yes, it begins in the mind / brain. The neurotransmitter stew in the brain that IS who we are is the source of much of what we FEEL both mentally and physically. With conditions like diabetes, some body part fails to do its job as planned. With diabetes, it's the pancreas. With hepatitis, it's the liver. Mental illness is the same. But with mental illness, it's the BRAIN -- arguably the most complex organ in the body.
So, the next time a bipolar tells you she's tired, have some compassion. It very likely has little or nothing to do with the condition you experience as TIRED. It isn't lack of sleep. It isn't excess physical or mental exertion. It is a very real inertia, not easily overcome. And it certainly should not be dismissed with the word JUST.