Originally Posted by RiPHRaPH

Wait. You are training for ~10 hours per week, correct? I would pay closer attention to what you are doing with the other 158 hours of the week. You are likely NOT overtrained, but not fully recovered.
You mention no age, nor fitness, nor time of day you are training. Any caffeine consumption?

I would first make sure you are adequately hydrated. Dehydration robs you of efficiency, throws off your electrolytes and makes your heart work harder.

First take a baseline HR. As you wake up in the morning for several mornings in a row, before your head is off the pillow take your resting heart rate using your clock radio. After several mornings you will get a feel for your true resting HR. Any deviation from this can tell you if you are overtrained.

I can't imagine a scenerio where someone can overtrain only exercising 10 hrs per week.
Drink till you urinate every 3 hours. You also don't mention where you live. If it is winter there, a humidifier will help you hydrate as well. You can lose up to 10% of your water through your respiration.

Your focus now should be steady base miles where you are not taking your HR up too high anyway. You don't need to be a January champion, but build so in late summer you've got something left (mentally and physically) in the tank.

Did you try to stretch to pre-fatigue your body before bed? No caffeine too late? Train earlier?