Sometimes the sleep indicator light/power indicator light may behave oddly. Apple wrote this support article to discuss the meaning behind those behaviors:
From the iMacs which look like:
Only the ones with an EMC# on the base 2104, 2105, 2110, 2114, 2118, 2111, 2133, or 2134 are Intel. The rest are PowerPC and have a different behavior.
Note the difference between this model, and the current silvery model. The earliest models with the white case had PowerPC inside, and the latter ones had an Intel inside before Apple shifted to the silvery case.