user21875: I have started facing a weird problem which I wouldn't earlier.
While meditating during daytime, I sometimes doubt whether I am actually experiencing nimitta or my vittaka has waned and, instead of making way for ekaggata, my closed eyes are in fact picking up ambient light.
This inevitably keeps me from attaining stable anapana-nimitta in that session and the progression plateaus, irrespective of how effortless maintaining concentration has become. That leads to restlessness and I either don't feel like keeping my aditthana, or if I can nullify that I emerge from the session somewhat disappointed that I didn't utilize the session well.
I'd like to clarify that I not seeking any jhanic factor or for anapana-nimitta to emerge, and I can attain anapana-nimitta and subsequently absorption for desired time period before progressing to the higher jhanas during night-time because I know I can't be confounded by ambient light...
So, other than only sitting at nighttime, which isn't really a solution, what do I do? Any pointers?

user-1: Whether it is or isn't a vision of light i can't tell and i would advise you not to care about the classfication. Rather just discern it's arising, persistence, change & cessation as one would in course of general kayagatasati development.
I guess you are disturbed by this because you think that if it is ambient light then it is distracting you from the perception of breathing but seeing ambient light isn't really a problem lest you make it a problem.
Did you know that according to the Sutta the particular benefit of anapanasati is the cutting off distracting thoughts rather than seeing lights & visions?

He should develop perception of unattractiveness so as to abandon lust... good will so as to abandon ill will... mindfulness of in-&-out breathing so as to cut off distractive thinking... the perception of inconstancy so as to uproot the conceit, 'I am. An9.1

If your inclination & bent is seeing white lights, then there is really no point in maintaining mindfulness of the breathing over a perception of diffused light lest the perception of diffused light is causing distracting thoughts.
Also the in & out breaths are technically the internal air element and if you contemplate them as such and make it your inclination, then you can be expected to attain absorbtion based on the air kasina.
Whereas if you use the breathing merely to calm down whilst generally tuning your mind to the perception of light, to a pleasant feeling, to the deathless element or whatnot, then it can go either way once one is sufficiently secluded and it's not easy to guess what turns out to be the dominantly effective inclination.

user43: As other answers hinted at, you having these worries and doubts about your experiences and what they mean, is a sure sign that you got too obsessed with the concept of progress.
This concept is just an idea in your head, a kind of hook you are on. Paradoxically, the only way to get to the next level is to drop this notion of levels and progress altogether.
Try and let go, and see what remains of your meditation.

user16297: For starters, be aware that anapana-sati (breath meditation remembrance) as defined in the suttas is quite a different practice.
As this is a forum about Buddhism in general, it would be helpful if you designate what tradition you're practicing "anapana". Most people I would think, would assume when you say 'anapana', you're referring to sutta based practice which is completely different.  I've done the style of anapana you're describing before, and have experienced those same problems and much more.
My advice is, if you want to continue that type of anapana you're doing now, there are Taoist systems that do a much better job of it (working with visual light, third eye, states of formlessness and frozen stupor). If you're doing 'anapana' and having all kinds of problems that and have lost any connection with physical breath, how can it even rightly be called anapana?
(my notes with detailed studies on how sutta defines anapana, and how later Buddhism redefine into something else)
https://lucid24.org/sted/16aps/index.html

user16635: The key phrase in your question is "I sometimes doubt [...]". The "I" in this phrase is your mind entering to question your meditation practice, because that's what the thinking mind does: it looks for problems to worry about so that it can keep itself active. There are three things you can do about it, depending on your inclinations:

Decide (for no reason whatsoever) that it is ambient light, and forget about it
Decide (for no reason whatsoever) that it is nimitta, and be content
Decide (for no reason whatsoever) that it is unimportant mind-stuff, and wait until it goes away

Whatever the case is will be self-evident once your mind gets out of the way, so feed, soothe, or ignore the thinking beast until it subsides.

user10100: We don't care about  nimitta or ambient light. We care only  the breaths.
The  nimitta or ambient light is naturally appearing at focusing the breath. Thinking of light or nimitta is not thinking of breaths. Thinking out of breath is losing the meditation. Focus on the breaths. Don't think of  nimitta or ambient light, although it is appearing so shiny.
When the meditation on breaths are in balance and powerful, the  nimitta or ambient light will be stable and powerful at the breath together automatically without any attempt on them.

21875: <p>I have started facing a weird problem which I wouldn't earlier.</p>
<p>While meditating during daytime, I sometimes doubt whether I am actually experiencing <em>nimitta</em> or my <em>vittaka</em> has waned and, instead of making way for <em>ekaggata</em>, my closed eyes are in fact picking up ambient light.</p>
<p>This inevitably keeps me from attaining stable <em>anapana-nimitta</em> in that session and the progression plateaus, irrespective of how effortless maintaining concentration has become. That leads to restlessness and I either don't feel like keeping my <em>aditthana</em>, or if I can nullify that I emerge from the session somewhat disappointed that I didn't utilize the session well.</p>
<p>I'd like to clarify that I not seeking any <em>jhanic</em> factor or for <em>anapana-nimitta</em> to emerge, and I can attain <em>anapana-nimitta</em> and subsequently absorption for desired time period before progressing to the higher <em>jhanas</em> during night-time because I know I can't be confounded by ambient light...</p>
<p>So, other than only sitting at nighttime, which isn't really a solution, what do I do? Any pointers?</p>
 -1: <p>Whether it is or isn't a vision of light i can't tell and i would advise you not to care about the classfication. Rather just discern it's arising, persistence, change &amp; cessation as one would in course of general kayagatasati development.</p>
<p>I guess you are disturbed by this because you think that if it is ambient light then it is distracting you from the perception of breathing but seeing ambient light isn't really a problem lest you make it a problem.</p>
<p>Did you know that according to the Sutta the particular benefit of anapanasati is the cutting off distracting thoughts rather than seeing lights &amp; visions?</p>
<blockquote>
<p>He should develop perception of unattractiveness so as to abandon lust... good will so as to abandon ill will... mindfulness of in-&amp;-out breathing so as to cut off distractive thinking... the perception of inconstancy so as to uproot the conceit, 'I am. <a href="https://www.accesstoinsight.org/tipitaka/an/an09/an09.001.than.html" rel="nofollow noreferrer">An9.1</a></p>
</blockquote>
<p>If your inclination &amp; bent is seeing white lights, then there is really no point in maintaining mindfulness of the breathing over a perception of diffused light lest the perception of diffused light is causing distracting thoughts.</p>
<p>Also the in &amp; out breaths are technically the internal air element and if you contemplate them as such and make it your inclination, then you can be expected to attain absorbtion based on the air kasina.</p>
<p>Whereas if you use the breathing merely to calm down whilst generally tuning your mind to the perception of light, to a pleasant feeling, to the deathless element or whatnot, then it can go either way once one is sufficiently secluded and it's not easy to guess what turns out to be the dominantly effective inclination.</p>
 43: <p>As other answers hinted at, you having these worries and doubts about your experiences and what they mean, is a sure sign that you got too obsessed with the concept of <em>progress</em>.</p>
<p>This concept is just an idea in your head, a kind of hook you are on. Paradoxically, the only way to get to the next level is to drop this notion of levels and progress altogether.</p>
<p>Try and let go, and see what remains of your meditation.</p>
 16297: <p>For starters, be aware that anapana-sati (breath meditation remembrance) as defined in the suttas is quite a different practice.</p>
<p>As this is a forum about Buddhism in general, it would be helpful if you designate what tradition you're practicing &quot;anapana&quot;. Most people I would think, would assume when you say 'anapana', you're referring to sutta based practice which is completely different.  I've done the style of anapana you're describing before, and have experienced those same problems and much more.</p>
<p>My advice is, if you want to continue that type of anapana you're doing now, there are Taoist systems that do a much better job of it (working with visual light, third eye, states of formlessness and frozen stupor). If you're doing 'anapana' and having all kinds of problems that and have lost any connection with physical breath, how can it even rightly be called anapana?</p>
<p>(my notes with detailed studies on how sutta defines anapana, and how later Buddhism redefine into something else)
<a href="https://lucid24.org/sted/16aps/index.html" rel="nofollow noreferrer">https://lucid24.org/sted/16aps/index.html</a></p>
 16635: <p>The key phrase in your question is &quot;I sometimes doubt [...]&quot;. The &quot;I&quot; in this phrase is your mind entering to question your meditation practice, because that's what the thinking mind does: it looks for problems to worry about so that it can keep itself active. There are three things you can do about it, depending on your inclinations:</p>
<ul>
<li>Decide (for no reason whatsoever) that it is ambient light, and forget about it</li>
<li>Decide (for no reason whatsoever) that it is <em>nimitta</em>, and be content</li>
<li>Decide (for no reason whatsoever) that it is unimportant mind-stuff, and wait until it goes away</li>
</ul>
<p>Whatever the case <em>is</em> will be self-evident once your mind gets out of the way, so feed, soothe, or ignore the thinking beast until it subsides.</p>
 10100: <p>We don't care about  nimitta or ambient light. We care only  the breaths.</p>
<p>The  nimitta or ambient light is naturally appearing at focusing the breath. Thinking of light or nimitta is not thinking of breaths. Thinking out of breath is losing the meditation. Focus on the breaths. Don't think of  nimitta or ambient light, although it is appearing so shiny.</p>
<p>When the meditation on breaths are in balance and powerful, the  nimitta or ambient light will be stable and powerful at the breath together automatically without any attempt on them.</p>
