If someone turns on a light suddenly while you’re sleeping, does it affect your sleep (before you wake up)? How about a loud sound, or a disturbing smell? Various studies have been done examining the effect that stimuli of the different senses can have on sleep. These articles will review research done on the senses of sight (visual), sound (auditory), touch (tactile), and smell (olfactory).
Visual Stimuli (sight)
For a study investigating how visual stimuli affect sleep, participants wore special welding goggles that were encased with lights capable of emitting intensities from a range of 0-3,200 mCD (microCandelas–a measure of light intensity). As a reference point, cell phone lights are 20-30 mCD, and laptop computer screens are typically 20,000-30,000 mCD. The light stimuli were presented for half-second durations in varying intervals during different stages of sleep throughout the night. Shortly after presenting the stimuli, the participants were awakened and asked about any thoughts or dreams they might have been having.
The researchers found that the stimuli increased visual imagery in light, non-dream sleep (stage 2 of Non-Rapid Eye Movement—NREM—sleep) but did not affect imagery for dream sleep (called REM). Interestingly, researchers also found that adjusting the light frequency to a rate similar to brain waves commonly found in REM led to the brain waves being produced in NREM sleep. This also increased the frequency of dreamlike reports from participants when they were awakened.
Auditory Stimuli (sound)
How does sound affect dream sleep? To investigate this, researchers created an auditory device that produced a 1,000 hz tone in half second durations at a fifty decibel level—a little softer than the level of a normal conversation. This stimulus was presented for a full minute after participants were in clear REM sleep for one minute. Participants were then awakened and asked a series of questions about anything they may have been thinking or dreaming. As a control condition, a second group of participants were awakened from REM after two minutes, with no auditory signal presented.
Researchers found that when the auditory tone was used, the patients’ eye movements were reduced in frequency and amplitude—which is significant, since the size and frequency of eye movements are thought to be correlated with the content of dreams. This relationship was supported by other findings in the study—the average percentage of imagery reports, visual noun count, and average vividness rating were all lowered in the experimental condition (auditory stimuli present) versus the control condition.
Other studies investigating the effects of sound on sleep have yielded some interesting findings. Short, loud 1,000 hertz tones induced eye movements in NREM sleep—possibly producing “dreams” in non-dream sleep. Research examining reactions to sound stimuli in the deepest stage of sleep—sometimes referred to as “slow wave sleep”—found that patients suffering from night terrors were prone to have an episode in response to a loud buzzer sound presented when they’re in slow wave sleep. Additionally, for those prone to urinary incontinence during the night, bladder contractions and involuntary bed-wetting could be induced by stimuli such as loud clicks or hand-clapping during slow wave sleep.
How effectively an auditory stimuli wakes a person up from sleep depends on the significance of the stimuli to the person sleeping. For example, people are more likely to wake up to the sound of their name (and wake up faster) than they are to someone else’s name or a generic sound. Interestingly, this effect is even more pronounced when waking up from REM (dream) sleep.
Tactile Stimuli (touch)
Research involving the sense of touch has generally been centered around pain—at what point does a pain threshold get crossed and people wake up? Do they incorporate pain into their dreams?
A study investigating these questions produced pain in their sleeping subjects by having them wear a broad-band blood pressure cuff just above either their left or right knee. Pain thresholds were established when the participants were awake to ensure they were not hurt during sleep by the pressure cuffs being over-inflated. After five minutes of REM sleep, the pressure cuffs were gradually inflated until the patients woke up and were asked to give dream reports. If patients did not wake up on their own, they were awakened after 5-10 minutes of REM sleep following the maximum pressure associated with their pain threshold, or a large body movement. As a control condition, subjects were awakened after 5-10 minutes of REM sleep with no pain stimulus applied.
There were several interesting findings from this study. How pain was incorporated into subjects’ dreams was generally a direct and accurate reflection of how the pain would be reported if they were awake—i.e., the pressure was on their leg and it hurt in the dream, just as it was in reality. How susceptible the participants were to dreaming about pain appeared to be related to whether or not they had a traumatic experience associated with pain in their past. When pain did become present in dreams, it often became a primary motivating agent and the dream became structured in such a way to try and deal with the pain as an unnecessary intrusion to sleep.
Other studies investigating pain and sleep have found that REM sleep and slow wave sleep have a much higher arousal threshold to pain from thermal stimuli on the skin than the light NREM sleep adults typically spend most of the night in. In fact, these arousal thresholds were so high they actually approached the level of subjects’ pain tolerance when they were consciously awake!
Olfactory Stimuli (smell)
Anyone who has awakened to the smell of coffee or a variety of breakfast foods being made can attest to the power of smell related to sleep. But do olfactory stimuli actually affect sleep in any way?
A study was done in which researchers investigated the effect of hydrogen sulfide (which smells like rotten eggs) and carbon dioxide (which is odorless but in small concentrations can produce a stinging sensation in the lining of the nostrils). The stimuli were delivered by using little tubes placed in front of the nose while subjects slept, and were presented during the light and slow wave phases of NREM sleep.
Surprisingly, the rotten egg smell did not produce arousals from sleep, even at significant concentrations. The odorless carbon dioxide, however, did produce increasingly frequent arousals from sleep as the concentrations became greater. The researchers theorized that this was because the brain processed the smell from the hydrogen sulfide differently than the stinging sensation of the carbon dioxide.
While rotten eggs may not have caused arousals from sleep, other studies comparing it to phenyl ethyl alcohol (which smells like roses) have found it to be associated with an increase in negative emotions in dreams. Conversely, the phenyl ethyl alcohol was associated with positive emotions in dreams. It’s believed that this is due to the olfactory processing of the limbic system—which plays a significant role in emotional processing—that is active during REM sleep.
What about aromatherapy? Researchers investigating lavender—a popular aromatherapy agent—have found that it increases slow wave sleep in both men and women and is associated with higher levels of vigor the following morning. For women, lavender is also associated with an increase in light NREM sleep, a decrease in REM sleep, and a decreased amount of time spent awake after the initial onset of sleep. Interestingly, these effects are opposite for men.