I found this passage in the colloquy 'Surrectio Matutina' from an old book containing colloquies by Erasmus, Vives and some Schottenius Hassus. The book is titled 'Familiaria colloquia, operâ Christophori Helvici D. & Professoris Giessensis olìm, ex Erasmo Rotterodamo, Ludovico Vive & Schottenio Hasso selecta'. The 'eò' is marked with a grave accent, i.e., it is an adverb, not an ablative of 'is'. It must be the adverb meaning 'to such a point, thereto', as in: 'Eò, unde discedere non oportuit, revertamur.' But I can't relate that meaning to this passage. My translation so far has been: 'Wake up now, in the evening [you] will be allowed to go back to bed early.'
This is the same eo which is usually coupled with quo in the following pattern:
Quo [a comparative], eo [a comparative].
This patterns translates in English with compr + compr ("the higher the better", "the more expensive the wedding is, more likely the couple will devorce).
Yet, Eo + comp. appears sometimes without the "correlative" or the first part (as being implied, or simply not there). As in Varro:
si est homo bulla, eo magis senex (If a man is a bubble, all the more so an old man).
To better understand what's going on in the passage, it might be helpful to fill the implied part (at least this is how I read this):
Surge modo!, [quo maturius [surges]] eo maturius licebit lectum petere. Wake up now, [the earlier you do so,] the earlier you will be allowed to go back to bed.
As brianpck notes in a comment, since English can be elliptical in the exact same way a handy translation would be "Get up now. You will be able to go back to bed that much earlier!
It should be noted, as demonstrated in this question about this quo ... eo pattern, that this pattern has alternatives like quo...hoc and others.