Clock signal control circuitry is disclosed along with a method for switching a clock between modes and a computer program product. The clock signal control circuitry is for receiving a clock signal from a clock signal generator and for outputting said clock signal to synchronous circuitry that is to be clocked by said clock signal. It comprises: an input for receiving mode switching signals indicating said synchronous circuitry is to switch between modes, said mode switching signals comprising a clock gating request signal indicating said synchronous circuitry is to enter a sleep mode during which said circuitry is not clocked and a wake up request signal indicating said synchronous circuitry is to enter an operational mode during which said circuitry is clocked; and is responsive to said clock gating request signal to gate said clock signal such that no clock signal is output to said synchronous circuitry and being responsive to said wake up request signal to output said clock signal to said synchronous circuitry. The clock signal control circuitry further comprises: a data store for storing a delay value; and delay circuitry for delaying switching of said clock signal between modes in response to at least one of said mode switching signals, said delay circuitry delaying said switching by an amount dependent upon said stored delay value.