You can always use a boost converter if you must run from a 1.5V battery. Eg. Pololu 3.3V Step-Up Voltage Regulator U1V11F3.
However tests show that for a low-power application the power used by the converter is substantially more than used by the processor in power-down sleep mode.
For example, I measured around 300 µA from the supply to the converter when the processor was asleep. Compare this to 4.5 µA when the processor is powered directly from 3.3V.
Now this is well within spec for the converter, which claims “Good efficiency at light load: <1 mA typical no-load quiescent current”. It was indeed well under 1 mA.
Another similar product JeeLabs AA Power board had somewhat better performance with the processor asleep: 36µA.
The conclusion I draw from this is to stick to straight batteries if power conservation is important, using less batteries and a converter draws a lot more current.