I found an article on mother.ly that said the following about babies crying when all their needs have been met:

""I came across a book called The Aware Baby by psychologist Aletha Solter Ph.D. which explained that crying isn’t how babies get their needs met; it’s also a healing process to recover from stress and upset.

Solter says:

“it is important to distinguish two main functions of crying. The primary function is to communicate immediate needs and discomforts...such as feeding, holding, a diaper change... or an extra blanket. The secondary function is that of the stress-release mechanism. Infants sometimes cry for long stretches of time even though they have no immediate need other than to be held and loved. They are releasing pent-up tensions through crying, and this helps them relax.”

Tears have been found to contain cortisol, the stress hormone, and research shows that crying lowers blood pressure and heart rate. Crying also activates the parasympathetic nervous system which acts as a self-soothing mechanism.

The idea that a baby could be ‘stressed’ can seem strange at first. However for babies, everything is new, and even things like having a shirt pulled over their head or going in a car seat can be stressful. Babies are just getting used to being in the world, and crying is one of the ways in which they process events and recover from overstimulation.""

I found this after weeks of frustration with my little girl's crying which seems to happen at the exact same time of day - everyday. Always an hour before bed.