Asp.net Why Thread.Sleep() is so CPU intensive ?

I have an ASP.NET page with this pseduo code:

while (read)
{
   Response.OutputStream.Write(buffer, 0, buffer.Length);
   Response.Flush();
}

Any client who requests this page will start to download a binary file. Everything is OK at this point but clients had no limit in download speed so changed the above code to this:

while (read)
{
   Response.OutputStream.Write(buffer, 0, buffer.Length);
   Response.Flush();
   Thread.Sleep(500);
}

Speed problem is solved now, but under test with 100 concurrent clients who connect one after another (3 seconds lag between each new connection) the CPU usage increases when the number of clients increases and when there are 70 ~ 80 concurrent clients CPU reaches 100% and any new connection is refused. Numbers may be different on other machines but the question is why Thread.Sleep() is so CPU intensive and is there any way to speed done the client without CPU rising ?
I can do it at IIS level but I need more control from inside of my application.

Answer is:

Just a guess:
I don't think it's Thread.Sleep() that's tying up the CPU - it's the fact that you're causing threads to be tied up responding to a request for so long, and the system needs to spin up new threads (and other resources) to respond to new requests since those sleeping threads are no longer available in the thread pool.