Sensory Processing Disorder (SPD) describes a condition where the brain has difficulty receiving and responding to information that comes in through the senses. This neurological difference can lead to sensory seeking (under-responsive) or sensory avoiding (over-responsive) behaviors in response to everyday stimuli. Many individuals with SPD report a noticeable increase in distress or heightened sensitivity during the late afternoon and evening hours. This fluctuation suggests that the nervous system’s ability to manage sensory input is not constant throughout the day. Biological, physiological, and environmental factors reveal specific mechanisms that make the transition to nighttime particularly challenging for the sensory system.
Circadian Rhythms and Changing Sensory Thresholds
The body’s internal 24-hour clock, known as the circadian rhythm, profoundly influences neurological function and sensory thresholds. This biological timing system directs cycles of alertness and rest by regulating the release of various hormones and neurotransmitters. As the day ends, the body naturally begins to lower its state of arousal in preparation for sleep.
One significant shift involves the stress hormone cortisol, which typically peaks in the morning and decreases throughout the day. For individuals with SPD, this natural drop in cortisol leaves the nervous system with less defense against stress and overstimulation. Simultaneously, the pineal gland secretes melatonin, the hormone that signals the onset of biological night and promotes sleep.
This neurochemical transition to a lower-arousal state makes the nervous system more reactive to stimuli it might have successfully filtered during the day. Sensory perceptual ability, including somatosensory and proprioceptive sense, fluctuates diurnally, with thresholds appearing more sensitive in the early evening. This heightened sensitivity is a biological phenomenon that lowers the tolerance level for sensory input as bedtime approaches.
The Impact of Accumulated Sensory Input and Fatigue
Worsening symptoms at night are heavily influenced by the duration of sensory exposure throughout the day, leading to sensory “burnout.” Every sight, sound, touch, and smell requires extra effort for a nervous system struggling with efficient processing. Filtering and regulating this constant stream of input depletes the system’s capacity for self-control and coping.
By the late afternoon, regulatory resources are exhausted, significantly reducing the ability to manage minor irritations. A small noise or uncomfortable texture that was tolerable earlier may now trigger an overwhelming reaction. This accumulated fatigue diminishes the neurological ability to modulate input, leaving the sensory gates highly reactive.
The decrease in coping mechanisms can manifest as sensory-avoiding behaviors, such as irritability or withdrawal, because the individual is too tired to process more input. Conversely, some may exhibit increased sensory-seeking behaviors, like hyperactivity, as the nervous system demands intense input to feel organized. This exhaustion creates a vicious cycle where dysregulation leads to poor sleep, compounding sensory difficulties the following day.
Environmental Shifts That Heighten Sensitivity
The changing environment as the sun sets introduces specific external factors that exacerbate SPD symptoms. The reduction in natural light forces the sensory system to rely more heavily on other senses, making previously masked stimuli suddenly prominent. A dark room removes the visual anchors used for spatial awareness, placing greater demand on the vestibular (balance) and proprioceptive (body awareness) systems.
The decrease in general daytime background noise allows specific, low-level sounds to become acutely noticeable. A refrigerator hum, the drip of a faucet, or the distant sound of plumbing can suddenly become distracting or painful for someone with auditory hypersensitivity. The quiet of the evening eliminates the white noise that previously masked these distinct sounds, making the auditory environment less predictable.
Changes in tactile input are unavoidable during the bedtime transition, which can be highly dysregulating for those with tactile defensiveness. The shift from daytime clothes to pajamas, the feel of new bedding, or the presence of a blanket can introduce intolerable textures and pressure points. These seemingly simple environmental changes transform the bedroom from a place of rest into a landscape of sensory challenges.
Practical Approaches to Improve Nighttime Comfort
Developing a predictable, sensory-friendly bedtime routine is the most effective strategy for managing heightened evening sensitivity. Consistency signals to the nervous system that a shift in activity is imminent, helping to lower arousal and anxiety. This routine should begin 30 to 60 minutes prior to the intended sleep time to allow for a gradual transition.
Proprioceptive Input
Incorporating proprioceptive input is a powerful way to calm the nervous system by providing deep pressure that releases relaxing neurochemicals. Activities such as “heavy work,” like pushing a laundry basket or performing wall push-ups, help ground the body and reduce seeking behaviors. Weighted blankets or compression sheets provide continuous deep touch pressure during sleep, leading to a quicker onset of sleep and better maintenance throughout the night.
Environmental Management
Managing the sensory environment involves careful control of light and sound. Dimming lights and switching to warmer color temperatures reduces visual stimulation, aiding natural melatonin production. For auditory sensitivities, using a white noise machine or soft, rhythmic music creates a consistent soundscape that blocks out sudden noises. Ensuring bedding and pajamas are made of preferred, soft materials addresses tactile sensitivities. The room temperature should be comfortably cool (around 65–70°F) to support the nervous system.