You need to make that determination with your doctor. It's usually either a Beta Blocker or Calcium Channel Blocker.

Your heart has an SA Node, that's where the heartbeat originates from. There's also an AV Node, it Discharges in response to the SA Node Discharge. But some people with age develop problems and their SA Node doesn't Discharge often enough.

If the SA Node fails to Discharge, eventually the AV Node Discharges on it's own. But in that case, the heart rate is VERY slow. But, not only can the SA Node Failing to Discharge properly cause a Slow Heart Rate, but issues with the AV Node can prevent a normally Discharging SA Node from passing through the AV Node rapidly enough.

There's various factors that determine Heart Rate, many of them are quite normal. That's why there's a range of normal Heart Rates at rest for various individuals.

In my case, my Heart is in Afib, the Atrium of my heart is beating very fast, but it can't get to the bottom of the Heart except (largely) going through the AV Node. Calcium Channel Blockers or Beta Blockers are used to slow the rate of which the very fast Atrium Discharges can make it across the AV Node, and they do it by influencing different things particular to their pharmaceutical properties.

I'm taking the maximum dose, by prescription, recommended of Metoprolol (a Beta Blocker) just to get the amount of control I have, 25 mg of Atenolol would not work for me. Talk to you doctor about your needs. I looked, it seems that most people are instructed to take Atenolol (and other Beta Blockers most likely) in the morning. Your slowest Heart Rate is in your sleep. Taking Atenolol in the morning will be during the time most people's Heart Rates are higher, and throughout the day, but the effects of the Atenolol begin to wear-off as the day progresses, so by bed time the Heart Rate would be faster than if you took Atenolol in the evening.

Hope this helps. Don't get caught up in the SA and AV Node thing, just trying to show various normal and abnormal factors influence resting Heart Rates.