My experience and knowledge of this subject doesn't compare to many on this thread and I defer to and respect (now and in the past) those sharing their thoughts. That said I'd like to share my experience.
I learned quickly that when I sleep around 8,000' or higher I suffer from very uncomfortable abnormal breathing. My symptoms are exactly those described for Cheyne-Stokes Respirations - "Above 3,000 metres (10,000 feet) most people experience a periodic breathing during sleep known as Cheyne-Stokes Respirations. The pattern begins with a few shallow breaths and increases to deep sighing respirations then falls off rapidly even ceasing entirely for a few seconds and then the shallow breaths begin again.
During the period when breathing stops the person often becomes restless and may wake with a sudden feeling of suffocation. This can disturb sleeping patterns, exhausting the climber.
This type of breathing is not considered abnormal at high altitudes. Acetazolamide is helpful in relieving this periodic breathing."
http://www.traveldoctor.co.uk/altitude.htm I also learned from personal experience that for me Diamox alleviates that periodic breathing pattern. Through this forum I learned about Diamox, discussed it with my doctor, and typically take 125mg twice a day starting a couple of days before sleeping at altitude. I then can sleep without those breathing difficulties. (I still am usually restless in anticipation of the next day's hike.)
Six weeks ago I hiked up the MWMR as a day hike. I arrived at Whitney Portal the day before, slept in my car, and was on the trail at 3:30 AM. This hike was a major goal of mine for the past couple of years. I had been part way up the MR three previous times familiarizing myself with the "route". I was in probably my best hiking shape, conditioning myself with this goal in mind. I am 65 and have been to the MW summit twice previously - once on the Main Trail as a day hike and once coming from Onion Valley camping several nights along the way.
I did succeed but was very disappointed in my progress ascending. I figured it would take me 7 1/2 hours to reach the summit. It took me 10 hours. My time to Iceberg Lake was OK - about 5 hours, but from there to the summit took me another 4 hours. While going up the chute I had to stop and gasp for breath about every 5 minutes or sooner. I found the chute to be steep with very loose rocky footing. I was advised to avoid the bigger chute in the center and take the smaller one to the left which I did. However maybe I should have been up out of the chute on more solid rock. I was mostly inside the chute searching for handholds so I could make my way up the loose rock.
While in the chute I made the decision to descend via the Main Trail. I did not want to ever be in that chute again. I was happy with my pace going down - it took me 6 hours.
Perhaps if I had spent an extra night or two at altitude (maybe Horseshoe Meadows) I would not have experienced the many debilitating bouts of sucking wind. Now that some time has passed I often consider a return trip to try again with better acclimation. I should mention I live in Reno and sleep every night at 5500'. In the past I figured this gave me an edge.
