A recent study suggests that a high-sugar, high-fat breakfast may reduce morning alertness and cognitive productivity.
The study was conducted at Hyogo University and published in the journal Food and Humanity.
What Did Researchers Study?
The study included 13 healthy female students. On separate days, they consumed two types of 500 kcal breakfasts:
- Balanced traditional Japanese breakfast – rice, fish, egg, vegetables, fruit
- Typical “fast” breakfast – donut and sweet milk-based drink (high in sugar and fat)
Researchers measured:
- Body temperature
- Heart rate variability
- Cognitive performance
- Subjective alertness and sleepiness
Understanding the Autonomic Nervous System
The autonomic nervous system regulates involuntary body processes:
- Sympathetic system → alertness, readiness for action
- Parasympathetic system → relaxation, “rest mode”
The balanced breakfast increased sympathetic activation, linked to better alertness and cognitive flexibility.
The sugary breakfast showed parasympathetic dominance, associated with greater sleepiness and weaker planning performance.
Why Can Sugary Breakfast Make You Sleepy?
- Sugar causes a rapid blood glucose spike, followed by a drop
- This fluctuation may lead to fatigue
- High-fat meals may slow digestion and affect energy stability
Although sugar can briefly increase dopamine (improving mood short-term), it does not support sustained concentration.
Practical Takeaway
For better morning productivity:
Choose:
- Protein (eggs, fish, yogurt)
- Complex carbohydrates (whole grains)
- Vegetables and fruit
- Healthy fats (nuts, avocado)
Important Notes
- Small sample size (13 participants)
- More large-scale studies are needed
- Individual metabolic responses vary
