Swedish researchers spent a year following elite athletes through their rigorous training schedules, and monitored how many of them suffered from injuries. The results showed that athletes who were hard on themselves when they had a bad day were the most likely to get injured, regardless of the severity of their schedules. The idea is that when you're blaming yourself for not doing your best work, you're more likely to push through any pain or discomfort you may be feeling, even when you risk getting injured.
Next time you're feeling sluggish at the barre during your morning class, or struggling with a combination that's usually no problem, try not to beat yourself up about it. Of course, it's important to challenge yourself, but everyone has an off day sometimes. Learning to accept that and act accordingly will make you a better, healthier dancer in the long run.