A lot of people live in a 7 day cycle, where 5 days of work are followed by 2 days of "rest". Vacations and holidays can increase the time available for resting.
Is there any biological reason for humans to rest after x days of work? Can this be explained by the buildup of by-products of metabolism, like Homocysteine?
I think your main assumption is not true -- we do not really rest on weekends.
I do not know many people who only rest on weekends. Usually this is the time for socializing, doing home-related work, physical activity (sports, hiking), spending time with your kids, all of which is often almost the opposite of a rest.
Our bodies work in cycles.. Circadian cycles that refer to our daily routines. We need sleep every night, eating at night affects us differently than if eating in the morning, etc... all because our bodies functions work in these 24 hour cycles... Now, beside the circadian cycles, our bodies also have circaceptan (or 7 day) rythms. For example, our immune system picks every 7 days (reason why the 7th day after a transplant is the most likely one may reject the organ). This 7-day cycle is also observed in other life forms... But, read for yourself: a seven-day cycle has been found in fluctuations of blood pressure, acid content in blood, red blood cells, heartbeat, oral temperature, female breast temperature, urine chemistry and volume, the ratio between two important neurotransmitters, norepinephrine and epinephrine, and the rise and fall of several body chemicals such as the stress coping hormone, cortisol. "In fact," Perry and Dawson note, "weekly rhythms appear easiest to detect when the body is under stress, such as when it is defending itself against a virus, bacterium, or other harmful intruder. For example, cold symptoms (which are really signs of the body defending itself against the cold virus) last about a week. Chickenpox symptoms (a high fever and small red spots) usually appear almost exactly two weeks after exposure to the illness.:" [Susan Perry and Jim Davson, The Secrets Our Body Clocks Reveal, (New York: Rawson Associates, 1988) p. 22.]
Doctors have long observed that response to malaria infection and pneumonia crisis peaked at seven days. Organ transplants face similar crises as the body's immune system attack the foreign organ. Campbell explains: "When a human patient receives a kidney transplant, there is a rhythm of about seven days, a predictable rise and fall in the probability that the body's immune system will reject the new kidney. A major peak of rejection occurs seven days after the operation, and when a serum is given to suppress the immune reaction, a series of peaks occurs, with increasing risk of rejection, at one week, two weeks, three weeks and at four weeks, the time of the highest of all." [Jeremy Campbell, Winston Churchill's Afternoon Nap, (New York: Simon and Schuster, 1986). p. 76]
So, in answer to your question... it makes a lot of sense that if so many of our body functions have 7-day rythms, we would feel a biological need to slow down and allow our bodies to rest, rejuvenate....
