Are there studies about the difference between meditating in the morning versus in the evening? Specifically, I am wondering if it is better to meditate at 7am or 7pm (even though doing both would be ideal).
The number of times or the time of day depends on the type of meditation that you do. If it is ‘Loving Kindness Meditation, it is good to do it for even a couple of minutes both morning, noon and night. That is what Ajaan Mun did as told to us by Thanissaro Bhikku. He said that,


  Ajaan Mun  spread thoughts of goodwill to all living beings three times a day: in the morning when he woke up, in the afternoon when he woke up from his nap, and at night before he went to sleep. In this way, the desire for goodwill, the desire for true happiness, framed his practice.


The best time to meditate is when you are not sleepy or drowsy or weighed down by the food. It can be in the morning or the evening. Most of the meditation practices that have become popular today are not in line with the Dhamma. Some ‘so-called teachers’ would erroneously advice a student of mediation to note in your mind, “pain, pain”, when you feel pain in some part of your body during meditation. For them it is vedananupassana – contemplation on feeling. Then your mind goes astray, they will get you to note “going, going”. Some view it as cittanupassana – the contemplation of mind. Likewise if sleepy, to note “sleepy, sleepy”, and, if a desire or agitation come to pass, to note “desire, desire”, or “agitation, agitation”. This to them is dhammanupassana – the contemplation of mind objects. These are incorrect practices that people are being taught as meditation. 

So it is not the time of the day as we do not know the nature of each individual. But no one should develop the idea of doing meditation in a hurry. Your goal should be to develop mindfulness, and not doing it in a hurry. So if you are more relaxed and have more time in the evening then that is the best time. Then do it little at a time and generate mindfulness. If too distracted or restless, then that is the time to stop. After generating mindfulness in that manner you will realize that you can maintain the object of meditation without becoming sleepy or without allowing the mind to get dispersed. Thereafter gradually increase the duration.