There is tremendous individual variation in what speed people acclimatize, at what altitude, and to what ceiling. The guidelines are like all guidelines, meant to be helpful for most people, not 100%. Some people are lucky, others not. Just like in all aspects of medical diagnoses.
Sleeping altitude is important for several reasons. Obviously one spends a lot of time in the tent, but more importantly what they are doing in the tent. Doing nothing? No, breathing while asleep is essential to life. During the day, you can voluntarily increase breathing, but at night, you depend on your autonomic nervous system, ie your brain, to sense and stimulate breathing while you are unconscious. It takes many days for your brain to get its central and peripheral oxygen and carbon dioxide sensors back in sync at the new altitude.
The above comments pertain to ventilation ( breathing), but other factors in acclimatization such as blood production and cellular metabolic changes take even longer, from many weeks to months, or some cases generations. For the average Sierra acclimatizer, the respiratory changes are the most likely as they are the ones that occur the quickest and in the time frame the person is visiting. The other changes take much longer, and occur in people who live at altitude, climb frequently, or stay for months. We are finding with more and more research in high altitude areas like Tibet that metabolic changes probably outweigh the others
