Every morning over the last 2-3 weeks our 1yr old BT bitch has decided to wake at around 5am

Initially we thought it was a once off event but sadly not.
We've tried ignoring the cries when she starts but she gets louder and continues. Once let outside she does pee & poo (as does our other BT). They are then put back to bed and around 1 hr later she starts again.
Nothing as changed in the dogs routine (meal times, bed time, pre bed toilet breaks etc).
Her last season (which was her first proper one) started back in late Oct and went on for some time. We are planning on getting her spayed shortly as her 'system' will have recovered from the season (the vets want 3 months after the season before they spay). Could this return to 'normal' be the reason?
Can anyone shed any light on how this can stop and believe me it has too as I am like a zombie due to the break in sleep
