STM32F103ZET6 ADC with DMA whilst sleeping and interrupt every 0.5s
Was an issue with the timer that I was using to trigger the ADC not being set to 1000Hz (1ms) and me not enabling both the half and full buffer interrupt.

Fixed Code:   

DMA_ITConfig(DMA1_Channel1, DMA_IT_HT, ENABLE); 
DMA_ITConfig(DMA1_Channel1, DMA_IT_FT, ENABLE);