I’ve been running and weight training for over 30 years. In my opinion, it doesn’t matter when you workout. Exercise is great, any time of day.
If you’re getting enough rest, your body feels good before and after training, and you’re able to perform your best, it’s okay to workout anytime. Having missed runs earlier in the day, I have gone running outdoors after midnight to make it up. For example, when I was on a night schedule at work, I have gone for quick, 3-4 mile runs at 12:30 am. At night, when subway service stops in my city, I frequently walk several miles home from bars and restaurants. I also weight train day or night depending on my schedule. The key is to workout consistently and frequently. For example, it’s better to run 2 miles a day, 3-4 days a week, than it is to run 8 miles once a week. Likewise, it’s better to workout during off-peak hours than miss a day.
When running outdoors, there are many benefits to going at night. Temperature and humidity usually drop, you eliminate the need to wear sunblock, there is less traffic, air pollution subsides, you can cross intersections without waiting for cars, there are fewer people (so you don’t have to feel embarrassed or uncomfortable if you don’t run fast) and the sidewalks are clear. In some towns, where parking is prohibited after certain hours, the roads are free of parked cars too. Of course use caution. This may not be safe or practical in every community.
You can weight train late at night too. Gyms are usually less crowded, you don’t have to wait for machines and equipment, there are fewer distractions, less traffic means it’s easier to get to the gym and you can save a lot of time. But weight training during off peak hours also limits the gyms you can use, restricts access to workout partners, trainers and coaches, and in some areas may pose a safety risk.
However if you’re training for a specific competition, it often better to train at the same time you’ll be competing. For example, if an upcoming race is scheduled in the early evening (like a Friday night fun run), you might want to get used to exercising and condition your body to be at peak performance at the same time. Elite marathon runners do this when they arrive in a host city ahead of time to familiarize themselves with the course and if necessary adjust to the time zone.
Regardless when you exercise, to be successful, you have to find times when you can consistently train several times a week. For cardio activities like running, frequency is usually more important than quantity. In my opinion, this is more important than trying to workout during “normal hours”. Scientific studies have also shown working out late at night does not have a signifiant impact on sleep. In other words, working out at midnight will not keep you up all night.