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By design intent, the high speed clocks (IMO, ECO, PLL, USB_CLK, MASTER_CLK and BUS_CLK and all derived clocks) are turned off to save significant current. The rule of thumb is that a running clock consumes about 100uA/MHz.
However, the ILO or the WCO can be left on in most low power modes.
If you don't need timing precision, you can use ILO @ 1KHz +100%/50%. It provide the LOWEST sleep current because the oscillation is at 1KHz.
If you need more precision you can use the WCO with the accuracy of the watch crystal you can solder on to P15 and P15. Note: The CY8CKIT-059 has 12pF caps already populated on P15 and P15. Just solder a 32.768KHz watch crystal across these terminals on the header. This is the second lowest sleep current since it oscillates at 32.768KHz.
Watch crystals can be found in very small SMD format and can be sub-$1 in small quantities. I use this option ALL the time.
PS: Using a low-cost watch crystal can also have additional benefits. For example, I use my WCO to retrim my IMO to make the IMO frequency MUCH closer to the target frequency than the PSoC's listed accuracy. For example, the IMO @ 3MHz is listed at +/-1%. I have gotten better than 0.04% when I use the 100PPM watch crystal as the reference for the IMO.