It could all be to do with the activity of a protein called BMAL1. Levels are often low first thing in the morning, and this can be the time when we find it more difficult to recall something, but as levels build during the day so does our ability to remember.
The proteins play a key role in the activity of the hippocampus, the area of the brain associated with memory.
The next stage could be to find ways of stimulating BMAL1 activity, say researchers from the University of Tokyo, as this could help dementia and Alzheimer's patients. The one thing they can't understand is why we seem to have naturally impaired memory when we wake up.