Overview
The 90-minute work block is a time management approach based on ultradian rhythms—natural 90-120 minute cycles that govern alertness and cognitive performance throughout the day. By aligning focused work sessions with these biological cycles and taking appropriate breaks between them, individuals can optimize productivity while maintaining sustainable energy levels.
The Science of Ultradian Rhythms
What Are Ultradian Rhythms?
Definition: Biological cycles shorter than 24 hours that regulate various bodily functions
Key Characteristics:
- 90-120 minute cycles
- Govern alertness and performance
- Occur throughout day and night
- Present in all humans
- Independent of circadian rhythm
Neurobiological Basis
Brain Function Cycles:
- Peak Phase (60-90 min): High alertness, focus, cognitive capacity
- Trough Phase (20-30 min): Recovery, decreased alertness
Neurotransmitters Involved:
- Acetylcholine (attention and focus)
- Dopamine (motivation and reward)
- Norepinephrine (alertness)
What Happens During Peak:
- Heightened focus capability
- Improved problem-solving
- Enhanced working memory
- Better information processing
What Happens During Trough:
- Natural fatigue signals
- Decreased concentration
- Mind wandering increases
- Body needs recovery
The 90-Minute Work Block Method
Structure
Work Block: 90 minutes of focused work
Break: 15-20 minutes of recovery
Repeat: 2-4 blocks per day
Why 90 Minutes?
Optimal Focus Duration:
- Aligns with natural alertness cycle
- Sustainable without excessive fatigue
- Long enough for deep work
- Short enough to maintain intensity
Not Too Short:
- Takes 15-20 min to achieve deep focus
- Need time to make progress after reaching flow
- 25-min Pomodoros interrupt flow prematurely for complex work
Not Too Long:
- Beyond 90-120 min, performance declines
- Diminishing returns on focus quality
- Increased error rates
- Mental fatigue accumulates
Implementing 90-Minute Blocks
Daily Schedule Example
For Maximum Deep Work (4 blocks):
- 8:00-9:30am: Block 1 - Most important deep work
- 9:30-9:50am: Break
- 9:50-11:20am: Block 2 - Secondary deep work
- 11:20am-12:00pm: Lunch and longer break
- 12:00-1:30pm: Block 3 - Collaborative/lighter work
- 1:30-1:50pm: Break
- 1:50-3:20pm: Block 4 - Tasks requiring focus
- 3:20pm onward: Shallow work, admin, wrap-up
Realistic Daily Capacity
Elite Performers: 3-4 high-quality blocks daily
Average: 2-3 blocks daily
Beginners: 1-2 blocks daily
Why Limited:
- Deep focus is cognitively expensive
- Quality decreases with more blocks
- Need recovery time between
- Diminishing returns after 4 blocks
Break Strategies
What Makes an Effective Break?
Physical Movement:
- Walk (ideally outside)
- Stretch
- Light exercise
- Change of posture
Mental Rest:
- No screens
- No work thinking
- Let mind wander
- Practice mindfulness
Social Connection (for some):
- Brief conversation
- Human interaction
- Not work-related
What to Avoid:
- Checking email
- Social media
- More cognitive work
- Staying at desk
Longer Breaks
After 2 Blocks: 30-45 minute break
- Meal
- Outdoor walk
- Exercise
- Power nap (20 min)
Purpose: Deeper recovery for subsequent blocks
Adapting to Individual Variation
Personal Rhythm Discovery
Track for 2 weeks:
- When focus naturally peaks
- When energy dips
- Optimal block length for you
- Break duration needed
Individual Factors:
- Age (older adults may need shorter blocks)
- Task complexity (harder work = shorter blocks)
- Experience level (experts sustain longer)
- Health and sleep quality
Flexible Application
80-Minute Blocks: For very demanding work
100-Minute Blocks: For flow-inducing tasks
Multiple 45-Minute Blocks: Alternative for fragmented schedules
Task Alignment
Matching Tasks to Blocks
First Block (Highest Quality):
- Most complex problem-solving
- Strategic thinking
- Creative work
- Learning difficult material
Middle Blocks:
- Important project work
- Writing and content creation
- Coding and development
- Analysis and research
Later Blocks (If doing 3-4):
- Less complex tasks
- Collaborative work
- Review and editing
- Planning and organization
Between Blocks (Shallow work):
- Email
- Administrative tasks
- Quick communications
- Filing and organization
Comparison to Other Methods
vs Pomodoro Technique (25/5)
Pomodoro Advantages:
- Easier for beginners
- More frequent breaks
- Less intimidating time commitment
- Good for simple tasks
90-Minute Advantages:
- Better for deep, complex work
- Allows achieving flow state
- Fewer interruptions
- Aligns with biology
Best Use: Combine—use Pomodoros for shallow work, 90-min blocks for deep work
vs 4-Hour Deep Work Blocks
Problem with 4-Hour Blocks:
- Exceeds sustainable focus duration
- Quality declines significantly
- Not aligned with ultradian rhythms
- Leads to burnout
Better Approach: Two 90-minute blocks with substantial break between
Scientific Support
Research Findings
Ernest Rossi (researcher who popularized ultradian rhythms):
- Documented 90-120 min cycles
- Showed performance decline after 90 min
- Demonstrated recovery need
Andrew Huberman (neuroscientist):
- Recommends 90-min focus blocks
- Emphasizes break importance
- Explains neurochemical basis
K. Anders Ericsson (expertise researcher):
- Studied expert performers
- Found 90-min practice sessions common
- Multiple sessions per day with breaks
Benefits
Productivity
- Higher quality work output
- Fewer errors and revisions
- Sustainable high performance
- More accomplished in less time
Well-Being
- Reduced mental fatigue
- Lower stress levels
- Better work-life balance
- Sustainable energy throughout day
Cognitive Health
- Prevents overwork and burnout
- Maintains long-term focus capability
- Respects biological needs
- Promotes recovery and restoration
Ideal For
Knowledge workers doing complex cognitive work, individuals seeking sustainable productivity, people experiencing afternoon fatigue, creatives and programmers needing flow states, anyone wanting to align work with biology, and professionals seeking to prevent burnout while maximizing output.
