Data
The numbers behind why people build second brains: memory, overload, the app market, AI adoption, and what happens when tools die. Every statistic links to its original source.
Last verified 2026-09-12 · 18 statistics · cite freely with attribution
In a 2015 replication of Ebbinghaus's forgetting curve, memory savings fell from about 56% twenty minutes after learning to about 22% after one day.
The replication, based on 70 hours of learning and relearning nonsense syllables, closely matched the curve Ebbinghaus published in 1885.
Source: Murre & Dros, PLOS ONE (2015)
Across three experiments, students who typed lecture notes on laptops performed worse on conceptual questions than students who took notes by hand.
Laptop note-takers tended to transcribe lectures verbatim rather than processing ideas in their own words, even with internet access disabled.
In a 54-participant EEG study at MIT, essay writers using ChatGPT showed the weakest brain connectivity of three groups and struggled to quote from essays they had written minutes earlier.
The preprint, which coined the term "cognitive debt", found writers using no tools exhibited the strongest and most distributed neural networks.
Source: MIT Media Lab, "Your Brain on ChatGPT" (preprint) (2025)
A 2025 study of 666 participants found a significant negative correlation between frequent AI tool use and critical-thinking scores.
The effect was mediated by cognitive offloading, and younger participants showed both higher AI dependence and lower critical-thinking scores.
Source: Gerlich, Societies (2025)
The average interaction worker spends an estimated 19% of the workweek searching for and gathering information, and another 28% managing email.
The McKinsey Global Institute report estimated that better internal knowledge tools could cut information-searching time by as much as 35%.
Source: McKinsey Global Institute, "The social economy" (2012)
The most heavily pinged fifth of employees are interrupted every two minutes during core work hours by meetings, emails, or chats, roughly 275 pings per day.
The figure comes from aggregated Microsoft 365 telemetry through February 2025 and covers the top 20% of users by ping volume.
More than half of 103 people surveyed in a Carnegie Mellon study felt they could not close a single browser tab, fearing they would lose information they had worked to find.
The CHI 2021 paper "When the Tab Comes Due" found people kept tabs open even while feeling overwhelmed or ashamed of how many they had.
Worldwide email users are forecast to grow from over 4.4 billion in 2024 to over 4.9 billion by 2028.
The forecast covers both business and consumer accounts, per the research firm's Email Statistics Report, 2024-2028.
Source: The Radicati Group (2024)
The global note-taking app market was valued at an estimated $11.02 billion in 2025 and is projected to reach $28.05 billion by 2030.
The 2026 report projects roughly 20.5% compound annual growth through 2030; estimates vary considerably between research firms.
Notion announced it had passed 100 million users in September 2024, roughly a decade after the company was founded.
Cofounder Ivan Zhao marked the milestone in a blog post tracing the product's growth from its first 1,000 users.
Source: Notion (2024)
Evernote reported some 225 million registered users at its tenth anniversary in 2018, a peak it never publicly surpassed.
By late 2022, when its sale was announced, the new owner would say only that the app had "millions of paying customers".
Source: The Register (2022)
OpenAI announced in October 2025 that ChatGPT had reached 800 million weekly active users, up from roughly 500 million at the end of March 2025.
Sam Altman shared the figure during OpenAI's DevDay keynote, weeks after the company reported crossing 700 million weekly users.
Source: TechCrunch (2025)
34% of U.S. adults said in early 2025 that they had used ChatGPT, roughly double the share who said so in 2023.
In the same survey of 5,123 adults, 28% of employed adults reported using ChatGPT for work.
Source: Pew Research Center (2025)
75% of knowledge workers reported using generative AI at work in 2024, with usage nearly doubling in the preceding six months.
The survey covered 31,000 people across 31 countries; 78% of AI users said they were bringing their own tools rather than using company-provided ones.
Mozilla shut down the read-it-later app Pocket on July 8, 2025, giving users until October 8, 2025 to export saves before permanent deletion.
The shutdown, announced in May 2025, ended a service that had operated for more than a decade and saved articles for millions of users.
Evernote restricted its free plan to 50 notes and one notebook effective December 4, 2023, about a year after Bending Spoons acquired the company.
The change followed the layoff of 129 Evernote employees in early 2023, with a spokesperson saying the app had been unprofitable for years.
Source: TechCrunch (2023)
Notion Mail shuts down on September 22, 2026, and drafts, scheduled emails, snippets, and auto-label rules not exported by September 21 will be permanently deleted.
Email history synced with Gmail is unaffected; only data created exclusively inside Notion Mail is at risk.
Source: Notion Help Center (2026)
Encrypted email and docs platform Skiff announced its shutdown when Notion acquired it in February 2024, initially giving users six months to migrate.
After user pushback, the migration window was extended to 12 months; Skiff accounts were not converted to Notion accounts.
Source: TechCrunch (2024)
Every figure on this page was checked against the linked source on the date above. Where a widely repeated number could not be traced to a verifiable origin, it was left out rather than repeated. Statistics age: always check the source year before citing, and treat market estimates as the named firm's estimate rather than settled fact.
You are welcome to cite any statistic here with attribution to its original source. If you spot an error or a broken link, tell us at [email protected] and we will fix it.
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