Like many, I can be grumpy if I have to wake up early. I didn't realise that it's not just the time of day - my genes might also affect my mood.
Scientists looked at the DNA of people with mood and sleep disorders. They found a change in a gene called PER3. The researchers realised PER3 probably plays an important role in the relationship between our mood and sleep-wake patterns.
Knowing this, I won't feel so guilty about my early morning bad moods.