I recorded the level of radiation in my apartment using a GMC-320+ overnight (roughly 8 hours) and awoke to find the graph below. Interestingly, both of the spikes took place at 55 minutes past the hour. What might have caused these spikes? I live across the street from both a Hospital and a Linear Accelerator, if that might help explain anything.
If you're really interested, take these measurements everyday over a month a you could notice a pattern. In any case, radiation spikes can come from a variety of things. K-40 reflecting off of walls, background cosmic, fancy "glow-in-the-dark" fluorescent objects that fluoresce on detecting gammas or alphas, lamps, bulbs, etc. Those spikes, however, can be very well strong x-rays (yousaid you were close to a hospital, that might explain a lot, or even if you're close to an airport, but you'd have to be really close). If you see a pattern, it could be from the hospital or some system. See if you can change the location of your counter. Take it to the living room, bathroom, set it beside a block of lead (if you have one) or next to concrete.