What causes itchy skin when being overtired? Is it just stress?

Worse at night. Itching is typically worse at night, because during the day you have your mind on more important things. At night when you go to bed, and have nothing in particular to think about, your itching becomes more noticeable.
Itchy skin. It stems from what the purpose of sleep is for your body. When you sleep, your immune system is going to work, repairing cells and healing your body. When your body is deprived of sleep, your immune system is weakened and probably didn't get a chance to do its job. Your skin is probably itchy because you may have a small allergy that does not effect you when you're fully rested.

Related Questions

I have had itchy skin for quite some time. When I scratch, I get raised welts that aren't red. Sometimes they are parallel lines. Stress?

Dermatographism. Your skin symptoms of itching or hyperreactivity can be due to allergy to something internal or external and may even be due to chronic stress. Antihistamines like Benadryl (diphenhydramine) or Claritin may help. Atarax has been helpful also. See an allergist if symptoms persist although sometimes a cause cannot be found.
Dermographism. Twitchy mast cells. A physical urticaria, not an allergy. Skin reacts to stroking. A topical steroid, probably prescription strength, to reduce itch will reduce scratching, thus reduce welting. Benadryl (diphenhydramine) and Atarax are very sedating and therefor appropriate only at bedtime. The best antihistamine for this is usually Zyrtec [cetirizine] which is sometimes used at 2 or 3 a day if 1 is not effective.

I have herpes & currently have an itchy skin rash on my neck & chest. Is it a result of herpes or something else? Been under a lot of stress lately.

Herpes. No. Herpes is not a rash that is extensive. It is a small very painful cluster of blisters. Now, reactivated Chicken Pox, or SHINGLES, can be a one sided extensive rash. But, it usually includes very painful blisters.