Central Nervous System Over-training in a nutshell.
Just like your muscular system, your central nervous system can be overworked. If you feel tired, weak, & unmotivated in your training, your sleepy CNS may be the cause.
What is CNS over-training?
Many people don't understand the difference between muscular & CNS over-training, & this confusion can allow them to fall prey to the latter.
Muscular over-training occurs when the muscular system is not given sufficient downtime to repair broken-down tissues.
Central Nervous System over-training is a systemic issue. It doesn't just impact the quads, chest, or back, but rather the whole body. Your CNS is responsible for generating muscular contractions in all types of training, so when you stack workout upon workout, eventually you can tire it out.
As a result of this general fatigue, you will be weaker & slower in all of your movements.

What causes CNS over-training?
In your training, the main cause is a program that has too much volume, too much intensity, & too little rest.
Remember: Even if you've planned your program to allow for enough rest between each session for a given muscle, your central nervous system is still being simulated.
All forms of stress can combine to overwork the system beyond its capacity. At that point, the workouts that normally sustain you may just be adding fuel to the fire & making you feel worse.
If you feel you are suffering from CNS over-training, try to take a comprehensive look at potential factors.
Solutions to stop CNS over-training.
Rework your workout program:
Allow adequate downtime between sessions working a particular muscle group, but also downtime between training sessions.
Sleep more:
Sleep is the body's prime tool for repair, so make sure you are getting at least 8 hours sleep each night.
Delegate:
Far too many people over work their schedules. when they really should say no to that additional task. You can only do so much. If that sounds simple, it's because it is.
Meditate:
Meditation, or even just deep breathing, is an excellent way to reduce your stress level, thus reducing the chance of CNS fatigue.