Based on two simple observations, I have produced a model of the body’s system of rest and energy. I have no training in biology, but I have noticed two strange facts of personal disposition.
1. A good night’s sleep does not assure me to feel good the next day
2. Exercise feels good and increasingly so with frequency
Many might assume the more sleep enjoyed the night before the better one will feel the next day. Through my experience this simpler observation does not hold. Or at least not so directly. Often it takes many days of consistently good sleep to produce an energetic state of body and mind.
It may not seem strange that exercise feels good (and by feel good I mean after it is done.) But in one sense, exercise is the very activity we should wish to avoid and minimize. Exercise requires more calories relative to the resting state. As a result pleasure in the activity signals a human to find more food than would otherwise be required. This is inefficient. Donning the evolutionary lens, food is scarce. A human that utilizes less calories to survive will outcompete one that requires more. So why would the body reward us for this wasteful behavior?
In summary, humans are constantly preparing for the unforeseeable stressful event. The world usually follows a mundane routine. However, occasionally (I won’t define a time period for “occasionally”) an unforeseeable crisis arises. This crisis requires great energy. If this energy threshold cannot be met death follows. You can never lose (at least not until you have fathered or birthed some children).
In order to prepare for the stressful event, the body maintains an energy reserve (adrenaline?). The energy reserve is a fixed cost that the body pays nearly every day. The energy reserve maintains the annoying property that it doesn’t store well. It needs to be continually replenished. It doesn’t go down to zero every day but continually needs a topping off.
Once the energy reserve has been satisfied the body feels free to release extra energy to the regular workings of the day. Now obviously this is a tricky relationship. The body doesn’t strictly release energy after the reserve has been satisfied, since this would mean regular operations couldn’t take place until the reserve is met. It’s more like the body delegates energy between regular activity and the reserve until the reserve is met. It is only when the reserve is met that the body releases extra energy. This release can be noticed through a positive attitude or natural exuberance.
The key here is consistency. In order to consistently feel “good” the reserve must be met every day and enough energy produced to consistently exceed the reserve. A good night’s rest for several nights will slowly improve a person’s natural mood. The fact that the improvement is slow implies that the energy reserve is a significant cost. Consistency is also good in that it allows the body to plan. Erratic rest may signal erratic times.
Where does exercise come in? Exercise can be thought of as an investment. During the process it feels terrible—the unavoidable cost. Shortly after, the body often releases a short term pleasure emotion. Usually if someone rarely or never exercises this short term benefit is largely outweighed by the short term cost. (The nature of these costs and benefits are not static.) But exercise also has a long term benefit. This is what an investment is: the exchange of a current cost for a long term benefit. Exercise challenges the body and in the processes improves the efficiency of energy production. Exercise is practice. It not only prepares the body for the future unforeseeable stressful event but also improves the body’s ability to produce the energy reserve along with the daily energy requirement for regular tasks. The short term pleasure signal, which seems strange, signals the mind to invest in the body.
The key here is also consistency. For simplicity sake let’s say that exercise costs net $1 today (to the body) and benefits $3 three days from now. If invested in every day the effective yield is a $2 benefit every day. The consistent benefit improves the incentive to continue exercising. In this way it can be thought of as a positive feedback cycle. This positive feedback serves as a subtler signal. "When I exercised in the beginning it hurt a lot, but felt good at the end. As I continue, it doesn't quite hurt as much and I am able to enjoy the benefit" (a stronger net benefit through time. If inconsistent the signal is much less clear. There is a net cost on the initial day of exercise rather than the net benefit experienced for the consistent exerciser.
This concludes my simple model. A fixed toll must be paid each day to the energy reserve. Exercise is an investment with a positive feedback cycle. This simple model has one simple policy prescription: if interested in feeling good, sleep and exercise consistenly. (Prescription might be good, but the model may be just pandering to it.)