The question of when to train is secondary to whether you train consistently, but the timing debate has genuine scientific substance. Research shows that body temperature, hormone levels, and neuromuscular function all fluctuate throughout the day, which can affect training performance by a measurable margin.
Morning workouts: core body temperature is lower in the morning, which can reduce initial performance but improves rapidly once you're warmed up. Testosterone levels are naturally higher in the morning, which theoretically benefits strength training. The biggest advantage of morning training is consistency - early sessions are less likely to be disrupted by work commitments, social obligations, or decision fatigue accumulating through the day. Many lifters find they think more clearly and feel more energetic all day after a morning session.
Evening workouts: body temperature peaks in the late afternoon and early evening, coinciding with peak strength and power output. Reaction time, grip strength, and aerobic capacity are all measurably higher in the afternoon and early evening. Many people find they can lift heavier and feel better training from 4-8pm than at 6am. The downside is disrupted sleep for some - intense late-night exercise can raise core temperature and stimulate the nervous system in ways that delay sleep onset. The evidence suggests evening training is slightly superior for performance, but morning training wins for consistency and real-world adherence. Train when you can do it most reliably.