Strength levels are higher when you train later in the day, but your sleep quality may be affected if you train then. Researchers had male and female subjects run on a treadmill for 30 minutes at 7 AM, 1 PM or 7 PM. When the subjects exercised in the morning, they spent 85% more time in light sleep, 75% more time in deep sleep and increased their sleep cycle frequency by 20%. In other words, the morning workout dramatically increased sleep quality. If you are having issues with sleep, consider switching your workouts to mornings.