Have you ever wondered why you felt so tired after lunch, a few hours after your first cup of coffee, or why consuming a maté or an energy drink in the afternoon keeps you up at night while your colleague can have a double espresso after dinner and sleep just fine? Sleep is a complicated and mysterious activity indispensable for any living organism. We humans are no exception, and we spend approximately one-third of our lives asleep. Nonetheless, most of us do not necessarily know why we do so, what makes us fall asleep, and how some substances that we may ingest regularly can affect our brains.
Different models and hypotheses have been put forward to explain why we sleep, but the most accepted one nowadays is the two-process model proposed by Borbély in 1982. The idea is that the interaction of two main factors influences our need for sleep. The first, process-S, stands for sleep drive and describes a need to sleep that increases with the amount of time an individual is awake. You might have noticed that if you have to wake up very early on a certain day, you feel more tired more quickly as the night approaches compared to a usual day. The neurophysiological chemical underpinning of this sleep pressure is called adenosine; during the day, the more and the longer our brain is active, the more adenosine it produces, and eventually, the sleepier we feel. Then, during the night, sleep allows a sort of “adenosine reset” by clearing out the adenosine that has been accumulated during the day. The second factor is process-C, which stands for circadian and is based on our sleeping habits and exposure to light throughout the day. The biological rationale underlying this process lies in the interaction of two hormones called cortisol and melatonin. Cortisol is secreted by the adrenal glands and influences our awareness and alertness. When you wake up in the morning, what enables you to get going and switch from the “sleepy mode” to the “aware mode” is a surge of this cortisol in your body, called the cortisol awakening response (CAR). This rise in cortisol levels in the morning is part of a daily fluctuation happening every 24 hours. It is then followed by a slow and regular decrease until the night when cortisol reaches its lowest levels. Conversely, melatonin, synthesized in the pineal gland, causes sleepiness. It is secreted at night in the darkness and is inhibited by light exposure. Melatonin levels also fluctuate throughout the day with an increase in the late evening before going to bed until approximately 2 or 4 a.m., when they reach their pinnacle, followed by a slow decline to very low daytime levels. Thus, the interaction between those two processes and their respective underlying chemicals regulates our sleep.
Caffeine is a psychoactive alkaloid usually classified as a central nervous system stimulant. You certainly already noticed that one of its main effects for most people is the increase in alertness and attentional capacity. One question that is often left untreated, though, is how it works, and whether it impairs sleep. First, caffeine acts on process-S of the two-process model described earlier. Indeed, we saw that the accumulation of adenosine during the day builds up some fatigue and a so-called sleep drive throughout the day. However, in order for this mechanism to work, the adenosine chemical has to bind to its receptor, just as a key has to go into a door lock to exercise its function. More precisely, caffeine acts as an adenosine receptor antagonist. This terminology might seem complicated but don’t worry, it is actually pretty simple. Consider you arrive home and want to open the door of your apartment (the adenosine receptor) with your key (adenosine), but you found that there is already a different kind of key (caffeine) in your lock, that can neither open the door nor be removed from the lock. As a result, you cannot open the door, but your key does not disappear. Now consider that the same is happening to your neighbors that are coming one after the other as time goes by, there is an accumulation of keys waiting to exert their function. Thus, once your body eliminates the caffeine (so when the key that is blocking the door is removed), your body will suddenly feel the sleep pressure indicated by all the adenosine that was “waiting behind the door”, but that could not bind to the lock because of the caffeine. This moment is sometimes referred to as a caffeine crash, especially if you were feeling tired (meaning you already had much adenosine present in the brain) before consuming your favorite caffeinated beverage. Now, to answer the question of whether caffeine impairs sleep or not, we have to consider that the half-life of caffeine ranges around 5 hours and that the body eliminates (or metabolizes) caffeine from the body in approximately 10 hours. This means that, for instance, if you ingest caffeine before 2 p.m., you will most likely be clean by the time you go to bed, and therefore caffeine will not interfere with your sleep. On the other hand, for most people, consuming caffeine later, especially at night, will greatly compromise their ability to go to bed early and their sleep quality in general. That is because caffeine will still be active in the body, preventing adenosine from doing its job. With that being said, caffeine sensitivity varies among people, and a small proportion of the population might be much less affected, at least subjectively, by caffeine consumption, even just before going to bed. Thus, the receptor sensitivity of these individuals certainly is not as pronounced as for the rest of the population. However, reduced sensitivity does not mean its complete absence. This implies that ingesting caffeine too late or in crazy amounts will probably still have some deleterious effects on sleep even if they are not perceived consciously.
To conclude, to feel rested, good sleep hygiene, sleep quality, and sleep quantity are the first parameters to focus on, as no amount of caffeine will ever be traded for this (the caffeine crash will serve as a good reminder of that). Then, if this is respected and you want to boost your energy levels or attentional capacities, there are ways to consume caffeine without impairing your sleep. If you are like most people, limiting your caffeine intake to the morning and early afternoon might be a wise solution. If you are one of those who do not seem to be affected by caffeine, a coffee later during the day might not be as compromising for your sleep, but moderation is still key.
Author: Pablo de Chambrier
