What is an itch? Why do we itch and what's the evolutionary reason? And why does scratching make it go away?

Sometimes when skin is itchy, it's because bacteria are making it itchy by being naughty moving around really fast to make you want to scratch it. What they do, is they dance around frantically on you and run and stuff moving really fast to make it itchy. When you scratch yourself, you are spreading the bacteria to more places so they stop making you itch now you've given them what they want as they can not infest you more.

There are number of reasons which can cause itching like Eczema, allergy,medical disorder,shaving,dermatological disorders etc. Basically release of histamines induce allergic and hypersensitive reactions,it initiates the sensation of itch and pain at sensory nerve endings as transmitter.

Studies using fUNCTIONAL magnetic resonance imaging (fMRI) have shown that scratching causes patterns of brain activity that are associated with pleasure and reward. This effect is strongest when you do the scratching yourself and when your are actually scratching and itch rather than just an arbitrary patch of skin. But contrary to what you might expect, or might have read somewhere, there's no clear evidence that this pleasure comes from a release of endorphins - it happens purely in brain and spine. This mechanism may have evolved us to encourage us to dislodge skin parasitises. All the evidence suggests that it's a very ancient response, because all vertebrates scratch themselves- even fish! Source - BBC Knowledge

This question is a little like asking “how long is a piece of string?”.The list of possible causes of itching is seemingly endless. For instance, just to name a few: Scabies Eczema Allergies Phosphate toxicity in renal failure Nerve damage Bird mites Dry skin Ingrown hairs Dermatitis Can you see what I mean?The fact that its occurring at rest quite likely means that that's the first time you're noticing it since you're not distracted.What does the 19/20 look like (I'm assuming you mean that there is some sort of rash or similar)?If it were me I'd try taking an antihistamine and see if that helps. Try finding a good veggie sorbolene and rubbing that on the skin. Change your washing powder to a sensitive one - use it for everything; sheets, towels, clothes. Try using low allergy soap, body wash, shampoo and conditioner.If none of that helps and it's really affecting you, is causing you harm or distress, or becoming significantly worse over time you should consult a doctor.

Various stimuli causes release of histamine which in turn stimulates the mechano-insensitive nocicepters. The impulse generated by these receptors are conveyed to the CNS via unmyelinated c- fibres, which are also the neurons for pain. Excessive scratching results in activation of pain stimuli masking the itch stimuli. Thus, we have a pseudosensation of relief from the itch.

You may have dermatitis or eczema, but you should see a dermatologist for evaluation. In the meantime, I’d get some hydrocortisone cream and apply it to itchy areas.

The sensation of light touch and sensitivity may provide an advantage to sense small things such as insects that carry disease or are venomous. That could certainly provide a survival advantage to mammals. However, that sensitivity can be triggered by benign things as well and is characterized at times as a disorder. To us, it can be annoying to disruptive. However, no one dies of an itch. Some dangerous medical problems can cause itchiness, such as Hodgkins Disease and people can die from the underlying disorder. However, you can die from malaria from certain mosquitoes so thIs sense is protective in many species. For clarity, I am not suggesting that humans evolved itchiness solely to help protect against insects. Light touch sense is likely a very old adaptation that enhances survival in many species. Also, the word purpose should be avoided. Evolution is a random process. A random change is preserved if it happens to provide a survival advantage that can allow passing on the genetic change. Some changes may provide no advantage but may be retained if there is no survival disadvantage.

this is not a diagnosis, but could be a simple place to start... assuming you have already eliminated things like soaps and chemical irritations, dry skin,  or allergies.... low blood calcium levels can cause body itch.  i have used Standard Process' calcium lactate with good results for this.  I only use SP these days because it is food sourced and a great product.  you would have to get it from a health care prof..... its not expensive....  some people sell it online but SP doesn't allow that and rumor is that much of the stuff sold illegally online are fakes... so they say....   easier to just get it from a health care prof and well worth a shot, after eliminating the first few things mentioned

They are never random. As all those before me said, there can be many reasons. But the point is that your nerve endings in your skin sense that perhaps a flea, or something moving is landing, basically anything that is irritating them and they signal your brain and you perceive it as a scratch. Scratching stops only if the reason for irritation is gone or you scratch so much your pain receptors are stimulated.