It's not unsafe to sleep on 30-40mg, it is just an indication that you're either really tired or you've developed a tolerance to methylphenidate. Although it is probably a combination of both.
Stimulants are safe if used as directed...most of the time. There is some debate concerning prescribed (theraputic) doses of the usually ADHD/ADD treatment stimulants (mixed amphetamine salts, dextroamphetamine, methylphenidate) are toxic when taken as directed.
Yes stimulants are addictive, there is no way around that.