What time should you head to the gym?

Most people will perform best in the evenings. However, people who often train in the morning will adapt to training at these times, resulting in similar performance and gains in strength and hypertrophy.

Arguments for why evening training may be include that our body temperature and thus heart rate and neural activity are higher in the evening, but this is a minor difference. There is no consensus on a difference in maximal muscle activity comparing morning to evening training.

Hormonally, the testosterone response is higher in the evening, but the basal levels are higher in the morning, while cortisol is lower in the evening. At the end of the day (pun intended), your performance seems to be the most important factor for controlling your muslce gains, rather than minor differences in hormone levels or body temperatue. Performance seems to be adaptable to the time of day in which training occurs for the majority of individuals. People who are chronically tired in the morning should, however, prioritize evening training.

Sources:
1. Chtourou H et al. The effect of training at the same time of day and tapering period on the diurnal variation of short exercise performances. J Strength Cond Res. 2012 Mar;26(3):697-708.
2. Souissi N et al. Effects of regular training at the same time of day on diurnal fluctuations in muscular performance. J Sports Sci. 2002 Nov;20(11):929-937.
3. Sedliak M et al. Effect of time-of-day-specific strength training on muscular hypertrophy in men. J Strength Cond Res. 2009 Dec;23(9):2451-2457.Source
Hayes LD et al. Interactions of cortisol, testosterone, and resistance training: influence of circadian rhythms. Chronobiol Int. 2010 Jun;27(4):675-705