Not sure if it helps- I used to do a lot of nightshift, finishing anywhere between 3am-6am. Hands down, it wreaks absolute havoc on your body. Even if you get the right amount of sleep, those hours just seem to mess with the body. I would feel like I was walking around in a dream/fog, and was very unproductive the day or so after. Once I got to sleep at a 'normal' time (for me, anywhere between 10pm-midnight) I'd feel better. Of course, there could be some mental stuff happening- perhaps the job isn't challenging you enough? What other things have you got in your life that give you a feeling of purpose? I find that without a clear purpose/goal/desire for something, the mental can also make you feel tired. You feel 'eh' as you describe. Not a professional at all, just my personal experience.