There are two answers to your question. Where Allah (SWT) & His Rasool has left the matter open for you you shouldn't ask for restrictions. [5:101] O you who believe, do not ask about things which, if disclosed, would displease you. If you ask about them while the Qur‗an is being revealed, they will be disclosed to you. Allah has pardoned you for it. Allah is Most-Forgiving, Forbearing. [5:102] People before you asked such questions, and then, as a result, became disbelievers. Other Ahadeeth and verses talk about the superiority of Dhik'r before sunrise and & sunset So you can apply this if you wish. [20:130] So, (O messenger,) endure with patience what they say, and proclaim the purity and praise of your Lord before sunrise and before sunset. And in some hours of night, proclaim His purity, and at points of the day as well, so that you may be pleased.