The phrase "to hit the sack" means to go to bed or to go to sleep.
Sentences with «to hit the sack»
Even drinking water too close to bedtime can interrupt sleep due to a full bladder, so avoid drinking anything at least two hours before hitting the sack. (wellnessnova.com)
• Staying awake for seventeen hours — getting up at 7 am and hitting the sack at midnight — leads to a decrease in performance equivalent to a blood alcohol level of 0.05. (womenshealthandfitness.com.au)
Improving your athletic performance may be as simple as hitting the sack earlier. (sleep.org)