Abstract
What did i do?
I've measured my sleep start time, sleep quality and quantity to find out what's my optimal bedtime.
How did i do it?
I've slept 145 nights with Dreem 2 headband and did a statistical analysis
What did i learn?
The later i'm going to sleep, the less total sleep time i'll get.
I've found my optimal sleep time around 21:30 - 22:30 and will stick to it.
Introduction
Going to bed at a right time seems to impact sleep quantity and quality according to recent scientific evidence. Current evidence indicate that about 7-8 hours of total sleep time (TST) is considered healthy optimal range for adults.
TST is a simply sum of Deep Sleep, Rem Sleep and Light Sleep.
The purpose of this observational data analysis (n=1) is to determine optimal sleep start time.
Materials & Methods
Participant
Adult male (n=1) with anthropometrics described below
| Mean | SD | n | |
| Age, y | 34 | ||
| Height, cm | 184 | ||
| Weight, kg | 60.2 | 1.44 | 348 |
| BMI, kg/m^2 | 17.7 | 0.42 | 348 |
| Waist, cm | 70.7 | 1.17 | 44 |
| Hip, cm | 86.1 | 1.13 | 44 |
| Neck, cm | 37 | 0.77 | 38 |
| Wrist, cm | 16.2 | 3 |
Weight was measured daily. Waist, hip and neck circumference - weekly. Wrist measure was not regular. n is number of measurements.
Experimental design
During 145 nights (from 2020-08-27 to 2021-08-21) sleep quality and quantity was assessed by Dreem 2 EEG Headband which was validated[1] against gold standard PSG.
Every sunday between 19:00 and 22:00 participant filled frequently used in research questionairres: General Anxiety Disorder (GAD-7), Center for Epidemiologic Studies Depression Scale (CES-D), Perceived Stress Scale (PSS), Fatigue Severity Scale (FSS), Insomnia Severity Index (ISI), Epworth Sleepiness Scale (ESS), Pittsburgh Sleep Quality Index (PSQI)
Everyday at the evening times between 19:00 and 22:00 Positive and Negative Affect Schedule (PANAS) was filled to asess PA and NA of the present day.
Every morning, within 10 minutes after waking up participant scored subjective sleep quality (SQ) on a 7-point Likert scale:
0 - Very poor,1 - Poor, 2 - Less than Avg, 3 - Average, 4 - Good, 5 - Excellent,6 - Perfect.
| Mean | SD | Median | IQR | 95% CI for mean | n | |
| PA | 29.1 | 3.71 | 30 | [27,32] | [28.6,29.5] | 287 |
| NA | 9.8 | 2.07 | 9 | [8,10] | [9.5,10.0] | 287 |
| ESS | 1.8 | 0.59 | 2 | [1,2] | [1.6,2.0] | 41 |
| GAD-7 | 4.1 | 1.48 | 4 | [3,5] | [3.7,4.5] | 45 |
| ISI | 7.0 | 2.53 | 7 | [5,9] | [6.3,7.8] | 45 |
| PSS | 10.4 | 2.94 | 10 | [9,13] | [9.5,11.3] | 43 |
| FSS | 20.9 | 5.03 | 20 | [17,24] | [19.2,22.7] | 35 |
| PSQI | 4.7 | 1.22 | 4 | [4,6] | [4.6,4.9] | 33 |
| SQ | 3.29 | 0.91 | 3 | [3,4] | [3.2,3.4] | 282 |
95% CI for mean was calculated using bootstrap percentile. PA, NA, SQ was assessed daily, everything else assessed weekly. IQR - interquantile range [Q1,Q3]
Results
The data summary shown below
TST = (DEEP + REM + LIGHT), SOL - sleep onset latency, WASO - time awake after sleep onset. StartTime, DEEP, REM, LIGHT, TST in hours, SOL in minutes
Linear regression
| effect | p-adjusted | effect size | |
| TST, h | -0.56 | 0.0003* | weak |
| DEEP, h | 0.074 | 0.55 | very weak |
| REM, h | -0.06 | 0.69 | very weak |
| SOL, m | 2.83 | 0.69 | very weak |
| WASO, h | 0.03 | 0.51 | very weak |
| Awakenings, count | 0.17 | 0.69 | very weak |
| Position Changes, count | 0.96 | 0.69 | very weak |
We can see a slope of -0.56 hour (-33 minutes) in Total Sleep Time for each additional 1 hour of Sleep Start Time with weak effect size. Even with weak effect size, -33 minutes seems to be valuable change. 95% Confidence interval for slope is very narrow [-0.568,-0.551]. Other sleep characteristics didnt show significant results.
Total sleep time (TST), hours
StartTime - 24h format sleep start time.
Discussion
The main result of this experiment is a statistically significant assocation between sleep time schedule and total sleep time. Since DEEP & REM stages arent affected, decrease in TST might by due LIGHT sleep.
In conclusion, the later i'm going to sleep, the less total sleep time i'll get.
Plot points me to optimal sleep times between 21:30 to 22:00 which i'm going to follow, seems there is no side effects.
A limitations:
- These results are NOT generalizable
- Absence of blinding, randomization, observational nature, n=1
- Often correlation ≠ causation
- Questionnaires widely used in research are prone to participant bias.
- There is a lot of factors affecting sleep (diet, exercise, caffeine etc) which not taken into account
Data availability & Information
Welcome for questions, suggestions and critics in comments below.
Dreem 2 device unmodified data is fully available here and format is explained by manufacturer.
Data for questionaires will be presented soon.
R Code and output is available on Github
RStudio version 1.3.959 and R version 4.0.2 was user for a simple linear regression model and to calculate slopes and p-values.
P-adjusted is p-value adjusted for multiple comparisons by method of Benjamini, Hochberg, and Yekutieli.
Effect sizes based on adjusted R2, Cohen's 1988 rules
References
- Arnal PJ, Thorey V, Debellemaniere E, et al. The Dreem Headband compared to polysomnography for EEG signal acquisition and sleep staging. Sleep 2020. https://doi.org/10.1093/sleep/zsaa097