Are there studies about the difference between meditating in the morning versus in the evening?

For certain individuals they may be alert more in the morning and others during the night. Even for the same individual this may change from period to period. So it is vise to divide the session into two parts.

Also more defilements gather when there are longer gaps.

So 1 hour in the morning and one in the evening / night is the minimum.

This little plant of Dhamma requires service now. Protect it from the criticism of others by making a distinction between the theory, to which some might object, and the practice, which is acceptable to all. Don’t allow such criticism to stop your practice. Meditate one hour in the morning and one hour in the evening. This regular, daily practice is essential. At first it may seem a heavy burden to devote two hours a day to meditation, but you will soon find that much time will be saved that was wasted in the past. Firstly, you will need less time for sleep. Secondly, you will be able to complete your work more quickly, because your capacity for work will increase. When a problem arises you will remain balanced, and will be able immediately to find the correct solution. As you become established in the technique, you will find that having meditated in the morning, you are full of energy throughout the day, without any agitation.

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When you go to bed at night, for five minutes be aware of sensations anywhere in the body before you fall asleep. Next morning, as soon as you wake up, again observe sensations within for five minutes. These few minutes of meditation immediately before falling asleep and after waking up will prove very helpful.

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Daily meditation of two hours and yearly retreats of ten days are only the minimum necessary to maintain the practice. If you have more free time, you should use it for meditation. You may do short courses of a week, or a few days, even one day. In such short courses, devote the first one third of your time to the practice of Anapana, and the rest to Vipassana.

Source: The Discourse Summaries by S.N.Goenka

The correct answer of course is: it depends.

The whole point of doing meditation is to become more aware of yourself, your thoughts, your body, your emotions.

So while meditating on a regular basis is a good idea I'd do it when you have the most time and are not in a rush. Also being aware is something you can do all the time by just watching and controlling your breath.

My experience is that if your day job is demanding morning meditation is better, you won't feel so tired and are less likely to fall asleep. I also used enjoy sitting in the midst of all the morning sounds, the birds singing, the bin men collecting the bins, people rushing off to work.

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.