Interference beetween ADC samples with XBee and photoresistor
Check the datasheet for the ATmega that is on your Arduino for 'ADC Noise Reduction mode`.


  To enter any of the six sleep modes, the SE bit in SMCR must be written to logic one and a SLEEP instruction must
  be executed. The SM2, SM1, and SM0 bits in the SMCR
  Register select which sleep mode (Idle, ADC Noise
  Reduction, Power-down, Power-save, Standby, or Extended Standby) will be activated by the SLEEP instruction.


Unfortunately, as far as I can see, you will have to write your own alternative for analogRead(); as the standard library seems not to support that the sleep command.

Check this article on arduino.cc for details on sleep.