Circadian variability 

Blood pressure (BP) follows a circadian rhythm, fluctuating in predicted patterns between night and day. These patterns include: 

• nocturnal dip – a substantial reduction in BP during sleep 
• rapid rise upon awakening
• increased variability during the daytime.  

The timing and amplitude of these BP fluctuations are influenced by intrinsic factors, with the sympathetic nervous system (SNS) being the primary contributor. The SNS modulates BP through the following mechanisms: 

• direct sympathetic neural input to the heart (from T1-T5 spinal level)  
• direct sympathetic neural input to the vasculature (from T1-T5 spinal level for the upper body and T6-L2 spinal level for lower body)  
• neurohormonal regulation, involving both the autonomic nervous system and renin-angiotensin-aldosterone system, which increase circulating levels of adrenaline, renin and aldosterone. 

In people with cervical and high thoracic spinal cord injuries, circadian blood pressure (BP) rhythms are frequently disrupted. Daytime BP tends to be lower than normal due to reduced sympathetic tone and decreased physical activity. At night, BP often resembles that of individuals with low-level paraplegia or those without SCI, eliminating the typical nocturnal dip in BP.

Extrinsic factors influencing circadian BP include:

• physical activity (the most significant daytime factor)
• emotional state
• smoking and/or substance use.

As physical activity levels are often reduced following SCI, this should be considered when interpreting circadian BP patterns.