Which formula correctly calculates the percentage of time spent in N2 sleep?

Which formula correctly calculates the percentage of time spent in N2 sleep?

• A. minutes of N2 / TST × 100
• B. minutes of N2 / TRT × 100
• C. minutes of N2 / TST
• D. minutes of N2 × TST

Answer: (A)

To find the percentage of time spent in N2 sleep, you take the minutes scored as N2 and divide by the total minutes of sleep (total sleep time, TST), then multiply by 100 to convert to a percentage. This ratio shows what portion of the whole sleep period was in N2. Why this works: TST represents all time spent asleep, across all stages. Using minutes in N2 as the numerator isolates the portion of that sleep time that was in stage N2, and multiplying by 100 gives you a percent. Using total recording time (TRT) would inflate the denominator with wake time and isn’t the right basis for a sleep-stage percentage. Skipping the ×100 would give a fraction rather than a percent, and multiplying minutes in N2 by TST would create meaningless units. Example: if N2 minutes = 28 and TST = 90, then (28 / 90) × 100 ≈ 31.1%.

To find the percentage of time spent in N2 sleep, you take the minutes scored as N2 and divide by the total minutes of sleep (total sleep time, TST), then multiply by 100 to convert to a percentage. This ratio shows what portion of the whole sleep period was in N2.

Why this works: TST represents all time spent asleep, across all stages. Using minutes in N2 as the numerator isolates the portion of that sleep time that was in stage N2, and multiplying by 100 gives you a percent. Using total recording time (TRT) would inflate the denominator with wake time and isn’t the right basis for a sleep-stage percentage. Skipping the ×100 would give a fraction rather than a percent, and multiplying minutes in N2 by TST would create meaningless units.

Example: if N2 minutes = 28 and TST = 90, then (28 / 90) × 100 ≈ 31.1%.