Do smoke detectors have radioactive stuff inside, and how far should you sleep from one?

No, they do not: have radioactive substances inside. The smoke detector should be in the room where you sleep. Here's a link to some information: http://www.nfpa.org/safety-information/for-consumers/fire-and-safety-equipment/smoke-alarms/safety-messages-about-smoke-alarms

Smoke detectors: do not emit harmful substances. They are classified as ionic or photoelectric detecting interruption in electricity or light by smoke. They should be located in the hall outside bedrooms--perhaps best one in each room--and on every household level...how far away from bed doesn't matter. Most important to install and maintain (check function/battery) regularly. It is VERY important. Good wishes:)

Yes they do !: One places these detectors on the ceiling or on a wall one foot down from the ceiling to be most effective (heat/smoke rises). So unless you hang from the ceiling to sleep, like batman, you are safe :)

Smoke Detectors: Good day; Smoke detectors are usually placed along the highest portion of the room. If not on the ceiling on a wall at a height adjacent to ceiling. Your bed or chair is normally found on the floor of the room in question on the opposite side of the room. Nothing more to worry about! Change your batteries annually!

Yes, but no danger: Yes, most smoke detectors contain a small amount of a radioactive isotope. But they are to be mounted on the ceiling, or high on a wall. Since smoke or other products of combustion (fire) are lighter than air, they rise to ceiling first and are detected (sounding alarm) before the concentration poses a threat at level where people are sleeping. There is no danger from device when 3 ft away.

Yes but Safe: The radioisotope used in a smoke detector emits a kind of radiation called "Alpha Waves" which are stopped by a sheet of writing paper - not the much stronger radiation you may be thinking of. The alpha source is contained in a metal box so no radiation can get out. Smoke detectors are completely safe in any room of your home. In the event of a fire their warning can save your whole family.

Ionization smoke: detectors expose people to a tiny amount of radiation—about 1/100 of a millirem per year. This is well below the background radiation level of about 360 millirems a year. If a smoke detector contains radioactive materials, a printed notice on the packaging will say so. There should be no harm since the detectors are usually on a ceiling or high on a wall.