A survey for the The Army Reserve shows 18-35s spend three-and-a-half hours a day on social media and gaming.
The survey of 2,000 is part of a recruitment campaign highlighting how young people wile away a day a week.
The Army Reserve said many young people think joining the reservists is too much of a time commitment - despite saying they would like to learn a new skill if they had more time.
The Army is aiming to reach a target of 30,000 trained reservists by 2019.
Reservists are required to commit to at least 19 days a year.
The study carried out on behalf of The Army Reserve by research specialists Opinion Matters asked 2,021 people, aged between 18 and 35, about their personal lives.
It found that more than 60% (1,213) said they would be interested in learning a new skill if they had more spare time, and a fifth (404) would like to volunteer and help others.
A spokesman for the Army Reserve said the research was designed "to get people who may have not considered the Army to think again about being a reservist and realise that it's not a huge commitment".
"With a time commitment from just 19 days a year, much of which is made up of short training evenings during the week, it's a realistic option for a lot of people and that the rewards are huge," he added.
The survey also suggested that:
- 37% (748) wanted improved fitness levels
- 38% (768) wanted to travel more
- 50% (1011) wanted to spend more time doing a hobby
By BBC defence correspondent Jonathan Beale
With cuts to the size of the regular Army, the country is having to rely more on part-time soldiers.
But the Army's struggling to meet its own target of 30,000 trained by 2019.
After a very slow start the numbers are growing, though the Army Reserve is still around 8,000 short.
After a series of high profile recruiting campaigns, already costing millions of pounds, the Army is adopting a high risk strategy.
It's now telling one of its key audiences - those aged between 18 to 35 - they're "wasting their time".
Rather than cause offence it's telling this generation they could be learning new skills and earning extra money as part-time soldiers rather than lounging on the sofa or logging into their social media accounts.
The most senior Reserve Officer, Major General John Crackett, says the key message is "what can the Army do for me?" with the slogan "A Better You".
Even he admits that meeting the recruitment target will be "challenging", but he insists it can still be achieved.