Try this weird little tip when doing crunches: put your tongue on the roof of your mouth.  It will help you align your head properly which will in turn help to reduce neck strain.  Give it a shot, it can't hurt!

This article is mostly about how a lack of sleep effect work performance, but you can see how this also relates to exercise.  Not only is sleep important to stay sharp and focused, but it is also when your body regenerates.

From Inc. Magazine:

The reason sleep is so important is because fatigue isn't simple. When we are tired, our performance doesn't degrade equally. Instead, when you lose a night's sleep, the parietal and occipital lobes in your brain become less active. The parietal lobe integrates information from the senses and is involved in our knowledge of numbers and manipulation of objects. The occipital lobe is involved in visual processing. So the parts of our mind responsible for understanding the world and the data around us start to slow down. This is because the brain is prioritizing the thalamus—the part of your brain responsible for keeping you awake. In evolutionary terms, this makes sense. If you're driven to find food, you need to stay awake and search, not compare recipes.

After 24 hours of sleep deprivation, there is an overall reduction of six percent in glucose reaching the brain. (That's why you crave donuts and candy.) But the loss isn't shared equally; the parietal lobe and the prefrontal cortex lose 12 percent to 14 percent of their glucose. And those are the areas we most need for thinking: for distinguishing between ideas, for social control, and to be able to tell the difference between good and bad.

Full article here.