Heat-assisted magnetic recording (HAMR) is expected to be a key technology to significantly increase the areal storage density of magnetic recording devices. At high temperatures, thermally induced noise becomes a major problem, which must be overcome in order to reliably write magnetic bits with narrow transitions. We propose an elementary model based on the effective recording time window (ERTW) to compute the switching probability of bits during HAMR. With few assumptions, this analytical model allows to gain deeper insights into basic noise mechanisms, like AC and DC noise. Finally, we discuss strategies to reduce noise and to increase the areal storage density of both bit-patterned and granular media.