How to Measure Your Nasal Cycle
This excercise concerns the "nasal cycle" -- to quote from the General Practice Notebook (a web-based medical encyclopedia):
"Airflow through the nasal passages is normally asymmetrical because of alternating changes in nasal resistance in each nostril. The mechanism involves changes in sympathetic tone to the venous erectile tissue of the nasal mucosa; increased sympathetic vasoconstriction causing resistance to fall. The total nasal resistance to airflow remains fairly constant as changes between the nasal passages tend to be reciprocal so that the patient is usually unaware of the phenomenon.....The reason for its existence is uncertain. A simple explanation is that it permits one side of the nose to go through a rest period and recover from the minor trauma of conditioning the inspired air."
For this exercise you'll need to carry around with you a small, clean mirror. You're going to hold the mirror underneath your two nostrils, so that it's just touching your upper lip. Leave it their as you normally inhale and exhale a few times -- don't change your pattern of breathing (see the figure on the left below). This will produce two "clouds" of condensation on the mirror, one associated with each nostril (see the figure on the right below). Judge which "cloud" is larger, left or right, and record that judgment along with the time of day you made it. Sometimes the judgment will be very easy (the differences will be obvious), but other times the differences may be subtle. Also, because the "borders" of the condensation cloud taper off gradually, you're going to have to adopt some criterion for what constitutes "the edges" of each cloud and you'll need to apply that criterion each and every time you make judgments.
Repeat this exercise every 20 minutes during a 12-hour period, so that you'll accumulate a total of 36 pairs of measurements. If it is impossible to make judgments at the appropriate time then do so as soon as possible after that and make a note when that judgment was made. After you've collected your data, plot the results in the form of a graph: time will be plotted along the horizontal axis and "larger nostril" will be plotted along the vertical axis (see example graph below). If you're like 80% of people, the graph will fluctuate over time (meaning that when one patch of condensation is small the other will be large, and vice versa). From this graph you should be able to extract your nasal cycle: this is the duration of time it takes for you to go through one complete cycle of diameter change.