Where do I start? Caffeine comes from a variety of sources; coffee, tea, chocolate, even guarana berries... but my particular focus here will be tea and coffee. Caffeine consumption is often overlooked, and not in the sense of awareness - as most of us are aware of the caffeine we consume - but in the widespread effects that it has on our mind and body.
On the positive side, it can increase alertness, memory, and performance. It can also suppress your appetite, proving useful in a phase of fat loss, and mask the signs of fatigue - which could serve as an aid, should you have something of importance to perform for. Many sources of caffeine are high in polyphenols and come at the cost of very few calories. Caffeine can increase mobilisation of fat for energy, which serves as a useful tool in a fasted state, and studies suggest it may promote autophagy - which I discuss at length in my book.
On the negative side, and here my argument weighs heavier towards coffee... the overconsumption of coffee amps up cortisol, the stress hormone. Over time, a tolerance can develop - leading to dependency - with cortisol steadily rising as you consume more for the same effect. Caffeine can impact sleep, cause anxiety, and also lead to digestive issues - especially when consumed on an empty stomach. As a natural diuretic, you may find yourself constantly dehydrated, which can provide the opposite effect to what you are seeking. Since caffeine provides a spike in felt energy, if this is not followed up with more caffeine (or a workout!), then a crash can often follow
How to keep caffeine on your side:
Consume more intense sources of caffeine, such as coffee, after roughly 3 hours of waking - as your body has systems in place to do that waking up for you! You also require more soon after to stoke the fire that you've started. Try switching out coffee for green tea, which contains the calming amino acid l-theanine. On occasions where you need to stay up late to work or study, turn to water to stay hydrated, this will allow you to stay awake and perform more effectively. Avoid overconsumption of caffeine on an empty stomach, as this can damage the lining of the gut. Finally, strategically place caffeine before a workout for its performance benefits - in my experience, this also eliminates the crash that follows and may also allow you to consume it slightly later in the day. Outside of this use, limit consumption after 3-4pm.