Blood tends to be thicker, or more viscous, in the morning due to a combination of physiological factors that occur overnight while you sleep:  

• Reduced fluid intake: While you sleep, you’re not consuming any fluids, leading to a slight dehydration. This decrease in overall body fluid levels can concentrate the blood, making it thicker.
• Hormonal fluctuations: Your body releases different amounts of certain hormones throughout the day and night: 
  • • Cortisol: This stress hormone naturally follows a daily cycle, with levels typically highest in the morning. Cortisol can influence blood pressure and heart rate, and some studies suggest it might also have a mild effect on blood viscosity.
    • Aldosterone: This hormone helps regulate blood pressure and fluid balance by controlling the levels of sodium and potassium in your body. Like vasopressin, it can influence blood volume, which indirectly affects blood thickness.
    • Melatonin: This sleep-regulating hormone also has antioxidant properties and may play a role in blood vessel health. While its direct impact on blood viscosity is not fully understood, some research suggests it could have a protective effect against blood clots.

  It’s important to note that the interplay of these hormones and their effects on blood viscosity is complex and still being researched. However, these hormonal fluctuations, along with reduced blood flow during sleep, are likely key contributors to the slight increase in blood thickness observed in the morning.

• Red blood cell production: Some studies suggest that the production of red blood cells may be slightly higher during the night, potentially adding to the blood’s viscosity in the morning.

While the increase in blood viscosity in the morning is usually mild and temporary, it can have implications for individuals with pre-existing heart conditions. Thicker blood is more prone to clotting, which can increase the risk of a heart attack, particularly in those with narrowed arteries.  

That’s why it’s important for individuals with heart problems to stay hydrated and take any prescribed medications as directed, especially in the morning. Additionally, adopting healthy lifestyle habits like regular exercise and a balanced diet can help maintain optimal blood viscosity and reduce the risk of heart complications.