Most of the work our brains do is involved in the task of shutting out distractions. It takes energy to crowd out distractions in order to focus on a narrow band of things. This exclusion process performed by the brain is the very nature of what we call focus and concentration. When we get tired, we run out of energy to keep shutting other things out so we lose focus. This loss of focus is also why most creative and inspired moments happen when we’re tired. For many of us, being tired is the only time we get to see the bigger picture.
It is important to afford ourselves some time to wander the streams of consciousness.