Presentation slides

Detailed research report

Motivation
Research shows that light of low wavelengths is harmful to human sleep. It is known that light of lower visible wavelengths, those in the range of 400nm to 500nm, reduces production of a hormone called melatonin in the human body. Reduction of melatonin in the blood increases wakefulness in human beings. Avoiding light sources that emit large amounts of light at lower wavelengths close to bedtime can improve sleep at night.

Objectives
The objective of this experiment is to identify the types of light sources that emit high intensities of low wavelength light that can affect sleep. The hypothesis is that light emitting diode (LED) lights of high color temperatures (called “daylight” type) will emit the maximum amount of light at lower visible wavelengths.

Approach
Different common light sources used in homes such as halogen lamps, compact fluorescent lamps (CFL) and LED lights of different power ratings and color temperatures and an iPhone SE in the normal mode and in the night-shift mode are evaluated by measuring (a) the relative intensity distribution of different wavelengths from each light source using a spectrophotometer, (b) the absolute light intensity emitted from the light source using a lux meter, and (c) the color temperature of the light source using a mobile app, and then calculating the proportion and total intensity of low wavelength (400-500nm) light emitted by the source using a software program written in Python. The measurements were made in a dark room in the late evening to eliminate the effects of any ambient light source. The experiment was repeated with light in a room illuminated by afternoon sunlight to be used as control. Three trials are performed for each measurement.

Results
Results show that the hypothesis is correct. The highest intensity of low wavelength light is produced by a 16W daylight (5262K) LED bulb. LED bulbs above 8.5W and CFL bulbs above 14W with color temperatures above 5000K are to be avoided around bedtime since they produce higher intensities of low wavelength light compared to that in a room illuminated by sunlight during the afternoon. Also, a 9W daylight (5225K) LED bulb is close to natural daylight in a room and can be considered good for reading in late afternoon.

Conclusions
Though LED bulbs are very energy efficient and are gaining widespread use, most of them other than those with low power ratings emit significant intensities of low wavelength light. These bulbs can affect sleep adversely and are not good for use close to bedtime.