I have tried a variety of exercise approaches in the morning, including endurance cardio (walking on the treadmill at around 3.6 mph for 20-30 minutes), alternating sprinting with walking, training with light weights for high reps (more than 15), and training with heavier weights for fewer reps (8-12).
One of the things I notice is which form of exercise gives me the most energy throughout the day. Intuitively, one might think that endurance training (moderately intense cardio for a long period of time) would be most effective in this regard. However, I’ve noticed that when it comes to having increased and lasting energy levels, it is actually training with heavy weights for few reps that is, for me, by far the most effective.
Though there is not much information I have found which addresses the effect of weight training on overall energy levels, there is a related study which might suggest a possible explanation.
The study recently compared the effects of high-intensity interval training and endurance training to determine which proved most effective in increasing cardiovascular fitness. High intensity interval training is alternating high intensity exercise (such as sprinting) with lower intensity exercise (such as walking).
The conventional wisdom has been that if you wanted to increase your endurance, you should increase the volume of exercise (run for a longer period of time, etc.). However, the study found that:
“… HIIT leads to similar and in some cases better improvements in shorter periods of time with some physiological markers. Incorporating HIIT (at the appropriate level of intensity and frequency) into a client’s cardiovascular training allows exercise enthusiasts to reach their goals in a very time efficient manner.” https://www.unm.edu/~lkravitz/Article%20folder/HIITvsCardio.html
I think something similar is occurring with higher intensity, shorter duration weight training. Training with heavier weights may force the body to adapt more than training with easier or lighter weights, even when one does more repetitions with the lighter weights. In other words, doing shoulder presses with 70 lbs. that you can do for a maximum of 8 reps may cause much more physiological adaptation than doing 25 reps with 30 lbs. I’ve observed this myself, in fact. Training with 70 lbs. will result in a much more noticeable change in musculature than higher reps with 30 lbs. Perhaps this increased adaptation also carries over to energy levels. Training in this way (interval training or weight training) may also may be less draining on energy reserves than endurance cardio.
This may not be everyone’s experience, however, it may be helpful to incorporate different types of resistance training into one’s fitness program to find out what gives the most optimal results.